Add Travis CI

This project has been [failing on Ruby 1.8.7 for 9 months](https://github.com/headius/thread_safe/issues/5)! I believe this problem would have been detected and fixed much earlier if this project used a continuous integration service.

This patch adds a `.travis.yml` configuration file and a badge to the `README` to make the build status more visible. All that is required after merging this patch is [enabling Travis CI for this project](http://about.travis-ci.org/docs/user/getting-started/).

I have also taken the liberty of doing up a few other things in the pull request:
* Added a note to the "Contributing" section of the `README` on how to build the jar and bundle
* Added a `rake` development dependency to allow `bundle exec rake`
* Cleaned up trailing whitespace

Each of these are in separate commits, so feel free to cherry-pick only the changes you want.

Also, please consider merging #7 when you have a moment, as that will make the build pass on Ruby 1.8.7.

Thanks!