Use org.codehaus.gmaven instead of org.codehaus.groovy.maven.

See http://jira.codehaus.org/browse/MNG-4863
Works with both maven 2.2.x and 3.0.x.
Merge pull request #221 from landryb/fix-gmaven-plugin

Use org.codehaus.gmaven instead of org.codehaus.groovy.maven.