Update grails to 2.3.1

Main changes:

Removed Spock dependency since this is included in 2.3.0.

Updated BuildConfig file to be parallel to Buildconfig that is generated when you create a new app.

Changed dependencies for tomcat and hibernate since they are now no longer versioned around grails versions.

Generate new grails wrapper. 

I had to change the SubnetControllerSpec as it was using ControllerSpec from the spock-grails plugin and causing a compilation error.

Also, the forked test mode causes an issue where CachedMaps don't seem to be able to be mocked. So I disabled this and will file a JIRA for it on Grails repo.