Update URL of JGit Maven release repository

This repository is required to allow clirr to compare the API of the
checked out version against the API of the latest release of jgit. The
old Maven repository on the download server was replaced by Nexus a long
time back.

Change-Id: I05125407fb72531c6831ec721064b0dad278bde5
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>