Add travis CI

Add Travis CI. This way, we don't have to worry about failing tests any more :)

If you edit .travis.yml and add the 3 items (GH_TOKEN, GRAILS_CENTRAL_USERNAME, and GRAILS_CENTRAL_PASSWORD) the script will automatically update the gh-pages branch with newly generated docs and publish a new version of the plugin (just does grailsw publish-plugin using the version directly and not modifying it).

I'm using this approach on a few other plugins. It's been a great help.

Make sure to go to https://travis-ci.org/profile/grails-plugins and enable Travis for grails-plugins/grails-cache or this change won't exactly do much :)