Switch to cobertura version available in Maven central

git-svn-id: https://svn.apache.org/repos/asf/creadur/rat/branches/gsoc@1548678 13f79535-47bb-0310-9956-ffa450edef68
Push cobertura report generation to top level

git-svn-id: https://svn.apache.org/repos/asf/creadur/rat/branches/gsoc@1548679 13f79535-47bb-0310-9956-ffa450edef68