added gs-geogit dependancy to geonode. (2.5-SNAPSHOT vs 2.5.1) 

since gs-geogit repo http://repo.opengeo.org/org/opengeo/geoserver/gs-geogit/ doesn't have 2.5.1 available, we can either peg it to 2.5-SNAPSHOT or ask boundless to add 2.5.1 / minor releases to the repo. I went with the first option. This dependency is needed so that the geogit section under layer overview which has already been committed works.