Use tox-travis to simplify .travis.yml (#56)

.travis.yml

 sudo: false  # run travis jobs in containers
 language: python
-cache:
-  directories:
-    - $PWD/wheelhouse  # cache wheels so that we don't hit network everytime
-env:
-  global:
-    - PIP_FIND_LINKS=$PWD/wheelhouse
-  matrix:
-    - TOXENV=py26
-    - TOXENV=py27
-    - TOXENV=py33
-    - TOXENV=py34
-    - TOXENV=pypy
-    - TOXENV=pypy3
+python:
+  - 2.6
+  - 2.7
+  - 3.3
+  - 3.4
+  - 3.5
+  - 3.6
+  - pypy
+  - pypy3.3-5.2-alpha1
+cache: pip
 install:
-  - pip wheel tox  # no wheel distribution for tox on pypi yet   
-  - pip install tox 
+  - pip install tox-travis
 script: tox

tox.ini

 [tox]
-envlist = py26, py27, py33, py34, pypy, pypy3
+envlist = py26, py27, py33, py34, pypy, pypy3.3-5.2-alpha1
 skipsdist = True
 
 [testenv]