add Magit hook to git-gutter:update-hooks

If the user has Magit, the buffer will update when a change is made.

If the user doesn't have Magit, then nothing changes.  Emacs will create
a hook magit-revert-buffer-hook that is never run.
Merge pull request #68 from jschaf/magit-hook

add Magit hook to git-gutter:update-hooks