Workaround JGit a deficiency in submodule updates.

JGit doesn't fetch from submodule's remote, so if you updated it to newer commit, 
you would get org.eclipse.jgit.errors.MissingObjectException on submodule update run.

git-submodule-update now fetches for you automatically.
version bump to 0.6.3
Merge branch 'master' of github.com:clj-jgit/clj-jgit