Travis CI: Drop EOL Pythons, Upgrade to Py 3.7, 3.8-dev, and current PyPy releases

Upgrade to the _production_ version of Python 3.7 in alignment with travis-ci/travis-ci#9069

Also see the note about outdated Python versions at:
* https://docs.travis-ci.com/user/languages/python/#development-releases-support

.travis.yml

@@ -8,24 +8,31 @@ cache: pip
 python:
   # Standard release https://docs.travis-ci.com/user/languages
   # /python#choosing-python-versions-to-test-against
-  - "2.6"
+  # - "2.6"  # EOL: https://devguide.python.org/#branchstatus
   - "2.7"
-  - "3.3"
+  # - "3.3"  # EOL: https://devguide.python.org/#branchstatus
   - "3.4"
   - "3.5"
   - "3.6"
-  - "3.7-dev"  # a.k.a "nightly"
-
+ 
   # PyPy2.7: https://doc.pypy.org/en/latest
   # /index-of-release-notes.html#cpython-2-7-compatible-versions
-  - pypy  # PyPy2.7 2.5.0
-  - pypy-5.1
-  - pypy-5.3
-  - pypy-5.4
+  - pypy         # Python 2.7.13 on PyPy 5.8.0
+  - pypy-6.0.0   # Python 2.7.13 on PyPy 6.0.0
 
   # PyPy3.3: https://doc.pypy.org/en/latest
   # /index-of-release-notes.html#cpython-3-3-compatible-versions
-  - pypy3.3-5.2-alpha1
+  - pypy3        # Python 3.5.3 on PyPy 5.8.0
+  - pypy3-6.0.0  # Python 3.5.3 on PyPy 6.0.0
+
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial    # required for Python 3.7 (travis-ci/travis-ci#9069)
+      sudo: required  # required for Python 3.7 (travis-ci/travis-ci#9069)
+    - python: nightly
+      dist: xenial    # required for Python 3.8-dev (travis-ci/travis-ci#9069)
+      sudo: required  # required for Python 3.8-dev (travis-ci/travis-ci#9069)
 
 install: "pip install -r requirements-test.txt"
 script: "py.test --doctest-modules boltons tests"