feature: Enabled travis-ci

I added support for travis-ci builds, as well as the build-status in the README.

To enable travis for this repo, you'll have to follow steps 1 and 2 here: http://about.travis-ci.org/docs/user/getting-started/#Step-one%3A-Sign-in.

I couldn't add a build for jruby, because of the native extensions in the fast_stack gem.