Enable pip cache in Travis CI

Can speed up builds and reduce load on PyPI servers.

For more information, see:

https://docs.travis-ci.com/user/caching/#pip-cache

Author: jdufresne

.travis.yml

@@ -1,4 +1,5 @@
 language: python
+cache: pip
 sudo: false
 install:
   - pip install tox