Added Travis CI support

This will allow pull-requests to be automatically run in a CI environment against multiple Rubies,
giving us extra confidence when reviewing and merging changes. This will however need a one-time configuration as described [here][1]:

I also removed the `Gemfile.lock` and added it to the `.gitignore`. See [here][yk] for the rationale.

[1]: http://docs.travis-ci.com/user/getting-started/
[yk]: http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/