Updated plugin to work with Grails 2.4:

- Update grails version
- Removed spock plugin (now bundled with grails)
- Added needed spring dependencies because of upgrade to Spring 4
- Refresh GenericApplicationContext _before_ use (otherwise Spring 4 blows up)
- Removed use of ConfigurationHolder
- Fixed applicationContext.xml for Grails 2.4
- Upgrade CXF to 2.6.6. Newer versions breaks testsuite