run travis in containers!

Travis CI's container based infra allows faster builds as it allows
caching, here change is made to use wheels and cache them so that
downloads do not hit the network once cached. Though the first build
takes a bit of time, subsequent builds should be effectively able to use
the build cache and do faster builds

.travis.yml

+sudo: false # Run travis jobs in containers
 language: python
+cache:
+  directories:
+    - $PWD/wheelhouse # cache wheels so that we don't hit network everytime
 env:
-  - TOXENV=py26
-  - TOXENV=py27
-  - TOXENV=py33
-  - TOXENV=py34
-  - TOXENV=pypy
-  - TOXENV=pypy3
-install: pip install tox
+  global:
+    - PIP_FIND_LINKS=$PWD/wheelhouse
+  matrix:
+    - TOXENV=py26
+    - TOXENV=py27
+    - TOXENV=py33
+    - TOXENV=py34
+    - TOXENV=pypy
+    - TOXENV=pypy3
+install:
+  - pip wheel tox  # No wheel distribution for tox on pypi yet   
+  - pip install tox 
 script: tox