Travis integration

I've added a simple .travis.yaml file to enable travis builds.

I've managed to get my fork build by travis with these settings: https://travis-ci.org/pschneider-manzell/grails-cors

Currently the plugin is tested against openJDK and OracleJDK with grails 2.1.1

Steps to activate Travis (after merging pull request which activates the test dependencies):
* Go to travis-ci.org and sign in using your GitHub account
* Go to "Accounts"
* Sync your Repositories
* Click on the "Settings" icon for "grails-cors" to activate Webhook URL notifying Travis of new commits
* Activate "grails-cors" repository by switching the button to "on"
* Wait until the first build passes

To do:
* After travis-ci is activated you can add a build status image to the main README.md