Updated to Grails 2.3.7

Dear Grailsteam,

I've updated the petclinic to use grails 2.3.7

I did:
- The upgrade itself
- Updated wrappers
- Changed tests to use spock
- Modeled the BuildConfig on one from a fresh 2.3.7 project
- Added Travis CI file, so I could test with a few JDK's and ensure that I did not have any "it works on my machine issues"

I hope you like the changes and will approve my pull request.

Best regards,

Sebastian