Grails 2.3 resolves a type in the class ClosureObjectMarshaller which previously was ClosureOjectMarshaller. Note missing b
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