Add Python 3.6 to tests and simplify .travis.yml by using 'tox-travis'

.travis.yml

@@ -1,31 +1,25 @@
 language: python
 
+python:
+  - pypy
+  - 2.7
+  - 3.3
+  - 3.4
+  - 3.5
+  - 3.6
 install:
-  - pip install tox codecov
-
-env:
-  - TOX_ENV=pypy
-  - TOX_ENV=py27
-  - TOX_ENV=py33
-  - TOX_ENV=py34
-# - TOX_ENV=py35
-# - TOX_ENV=flake8
+  - pip install tox-travis codecov
 
 script:
-  - tox -e $TOX_ENV
+  - tox
 
 matrix:
   fast_finish: true
 
   include:
-    # https://github.com/travis-ci/travis-ci/issues/4794#issuecomment-143758799
     - python: 3.5
       env:
-      - TOX_ENV=py35
-
-    - python: 3.5
-      env:
-      - TOX_ENV=flake8
+      - TOXENV=flake8
 
 after_success:
   - coverage combine && codecov

tox.ini

@@ -5,6 +5,7 @@ envlist =
     py33
     py34
     py35
+    py36
     flake8