Makes the grails-marshallers plugin compatible with Grails 2.3

Grails 2.3 resolves a typo in the class ClosureObjectMarshaller which previously was ClosureOjectMarshaller. Note the missing b
This pull request is more complete than the one by @jhsimms as it includes:
- Upgrades plugin dependencies to be fully Grails 2.3 compatible
- Removes Spock dependencies because Spock is now distributed with Grails
- Updates the plugin definition to specify Grails 2.3 ans the minimum version required.
- Upgrades documentation in Readme.md file to leave clear that:
  - For Grails version <= 2.2.x -> use marshallers plugin version 0.4
  - For Grails version >= 2.3.x -> use marshallers plugin version 0.5