Drop support for Python 2.6

It's about time this change is made. Python 2.6 is too old and if you
still need to use it, you would probably be facing bigger issues than
what is tool is designed for.

.travis.yml

@@ -3,7 +3,6 @@
 
 language: python
 python:
-  - "2.6"
   - "2.7"
   - "3.3"
   - "3.4"

setup.py

@@ -1,4 +1,3 @@
-import sys
 import re
 import ast
 
@@ -17,10 +16,6 @@ with open('./README.rst') as f:
 
 
 install_requires = ["pip >= 6.0.0"]
-if sys.version_info < (2, 7):
-    install_requires.append('argparse')
-    install_requires.append('ordereddict')
-    install_requires.append('importlib')
 
 
 setup(
@@ -34,6 +29,7 @@ setup(
     long_description=long_desc,
     install_requires=install_requires,
     extras_require={'graphviz': ['graphviz']},
+    python_requires='>=2.7,!=3.0.*,!=3.1.*,!=3.2.*',
     py_modules=['pipdeptree'],
     entry_points={
         'console_scripts': [
@@ -45,7 +41,6 @@ setup(
         'Intended Audience :: Developers',
         'License :: OSI Approved :: MIT License',
         'Programming Language :: Python',
-        'Programming Language :: Python :: 2.6',
         'Programming Language :: Python :: 2.7',
         'Programming Language :: Python :: 3',
         'Programming Language :: Python :: 3.3',

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py26, py27, py33, py34, py35, py36
+envlist = py27, py33, py34, py35, py36
 
 [testenv]
 commands =