Setup Travis-CI

It’s good to have visibility on the current status of tests on master and for pull requests.

Note that someone with admin rights in the repo will still have to do steps one & two in the [Travis-CI Getting Started guide](http://about.travis-ci.org/docs/user/getting-started/) to setup the GitHub hook.

Reasons for excluding various Ruby versions from running on CI:

- **1.8.7**: binding_of_caller dependency's native extension fails to build.
- **1.9.2**: Capybara requires Ruby 1.9.3+.
- **JRuby**: C extensions are not supported on Travis, but we need them.
- **REE**: binding_of_caller dependency's native extension fails to build.
- **Rubinius 1.8**: Nokogiri requires Ruby 1.9.2+.

Of course, some of the issues with later versions apply to the earlier versions as well.