Added -t option for `bundle gem` to generate a .travis.yml file with the current Ruby version

A lot of open source gems are running their tests on [Travis CI](http://travis-ci.org/). 

This PR adds the option to generate an minimal (but functional) `.travis.yml` config file, so you can start testing on Travis CI right away.

Let me know if you think we should make this a default file, and remove the option. Gems are *usually* open source, so it might be cool to encourage testing on Travis CI.