Add support for Travis CI

This adds a `.travis.yml` file for running tests with Travis CI.

Sample passing Travis CI build for my fork:

https://travis-ci.org/msabramo/python-memcached/builds/21521217

To set up Travis CI for your repo (takes less than 1 minute), see http://docs.travis-ci.com/user/getting-started/

Cc: @cabrera