Outdated git library doesn't support post-commit hooks

We run Zeppelin (0.7.2, on Amazon's EMR) and we tried adding git hooks, primarily to push the local notebook git repo to our remote. We noticed that post commit hooks don't get triggered. When I dug around, I noticed that pre-commits worked just fine.

Turns out that when git functionality was first implemented in late 2015, `jgit` (library used to provide all git-related functionality) was frozen at version 4.1.1, which you can see in the current `pom.xml`. However, `jgit` has gained post-commit functionality over a year ago, so Zeppelin could support it as well, if we updated the version of `jgit` that gets used. The current version is v4.9.0.201710071750-r (7 Oct 2017).