Add travis-ci configuration to run specs

To provide continuous integration to test the project against multiple versions of ruby.

Note these continuous integration builds will fail until issue #35 is fixed (because the current version of valid_email is not compatible with the latest versions of mail or rspec).

In addition to merging this pull request you'll need to configure the travis-ci web hook in github, see http://docs.travis-ci.com/user/getting-started/ for more details.