Use grails-dependencies artifact's Maven groupId coordinate to ensure that the correct version is used

The Grails version retrieval via the official Grails dependencies Maven artifact (org.grails:grails-dependencies) in the Maven project's dependency set should be more restrictive.  
I found that if I have another Maven artifact (internal artifact used for development purposes) with the artifactId of "grails-dependencies" in my Maven project then the version number from that artifact is used instead.  This change will ensure that the official Grails dependencies artifact's version will be used.