Use relative paths in pickled virtualenv files

Absolute path causes tests to fail on travis-ci.

tests/virtualenvs/cyclic_deps.pickle

@@ -30,7 +30,7 @@ sS'platform'
 p14
 NsS'location'
 p15
-S'/home/vineet/python/projects/pipdeptree/.tox/cyclic_deps/lib/python2.7/site-packages'
+S'.tox/cyclic_deps/lib/python2.7/site-packages'
 p16
 sS'py_version'
 p17
@@ -47,7 +47,7 @@ p22
 g16
 sS'egg_info'
 p23
-S'/home/vineet/python/projects/pipdeptree/.tox/cyclic_deps/lib/python2.7/site-packages/CircularDependencyB-0.0.0-py2.7.egg-info'
+S'.tox/cyclic_deps/lib/python2.7/site-packages/CircularDependencyB-0.0.0-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -80,6 +80,6 @@ p32
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 g16
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-S'/home/vineet/python/projects/pipdeptree/.tox/cyclic_deps/lib/python2.7/site-packages/CircularDependencyA-0.0.0-py2.7.egg-info'
+S'.tox/cyclic_deps/lib/python2.7/site-packages/CircularDependencyA-0.0.0-py2.7.egg-info'
 p34
 sbsba.

tests/virtualenvs/dummy.pickle

@@ -26,7 +26,7 @@ sS'platform'
 p12
 NsS'location'
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/src/lookupy'
+S'.tox/dummy/src/lookupy'
 p14
 sS'py_version'
 p15
@@ -41,7 +41,7 @@ p19
 g14
 sS'egg_info'
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/src/lookupy/Lookupy.egg-info'
+S'.tox/dummy/src/lookupy/Lookupy.egg-info'
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 sbsbag1
 (g2
@@ -63,7 +63,7 @@ S'2.5.2'
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 sg12
 Nsg13
-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages'
+S'.tox/dummy/lib/python2.7/site-packages'
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 sg15
 S'2.7'
@@ -76,7 +76,7 @@ p31
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/psycopg2-2.5.2-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/psycopg2-2.5.2-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -109,7 +109,7 @@ p41
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 g29
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/Werkzeug-0.9.4-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/Werkzeug-0.9.4-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -142,7 +142,7 @@ p51
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/Flask-0.10.1-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/Flask-0.10.1-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -175,7 +175,7 @@ p61
 g19
 g29
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/Mako-0.9.1-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/Mako-0.9.1-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -208,7 +208,7 @@ p71
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/MarkupSafe-0.18-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/MarkupSafe-0.18-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -241,7 +241,7 @@ p81
 g19
 g29
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/Flask_Script-0.6.6-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/Flask_Script-0.6.6-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -274,7 +274,7 @@ p91
 g19
 g29
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/alembic-0.6.2-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/alembic-0.6.2-py2.7.egg-info'
 p93
 sbsbag1
 (g2
@@ -307,7 +307,7 @@ p101
 g19
 g29
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/itsdangerous-0.23-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/itsdangerous-0.23-py2.7.egg-info'
 p103
 sbsbag1
 (g2
@@ -340,7 +340,7 @@ p111
 g19
 g29
 sg20
-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/SQLAlchemy-0.9.1-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/SQLAlchemy-0.9.1-py2.7.egg-info'
 p113
 sbsbag1
 (g2
@@ -373,7 +373,7 @@ p121
 g19
 g29
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/gnureadline-6.3.3-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/gnureadline-6.3.3-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -406,7 +406,7 @@ p131
 g19
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/slugify-0.0.1-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/slugify-0.0.1-py2.7.egg-info'
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 sbsbag1
 (g2
@@ -439,7 +439,7 @@ p141
 g19
 g29
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/Jinja2-2.7.2-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/Jinja2-2.7.2-py2.7.egg-info'
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 sbsbag1
 (cpip._vendor.pkg_resources
@@ -472,7 +472,7 @@ p151
 g19
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/ipython-2.0.0.dist-info'
+S'.tox/dummy/lib/python2.7/site-packages/ipython-2.0.0.dist-info'
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 sbsbag1
 (g2
@@ -505,6 +505,6 @@ p161
 g19
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-S'/home/vineet/python/projects/pipdeptree/.tox/dummy/lib/python2.7/site-packages/redis-2.9.1-py2.7.egg-info'
+S'.tox/dummy/lib/python2.7/site-packages/redis-2.9.1-py2.7.egg-info'
 p163
 sbsba.