Merge pull request #54 from edx/travis-updates

Using Docker and caching for Travis builds

.travis.yml

 language: python
 python:
     - "2.7"
+sudo: false
+
+# Cache the pip directory. "cache: pip" doesn't work due to install override. See https://github.com/travis-ci/travis-ci/issues/3239.
+cache:
+  directories:
+    - $HOME/.cache/pip
 before_install:
     - "export DISPLAY=:99.0"
     - "sh -e /etc/init.d/xvfb start"