add Magit hook to git-gutter:update-hooks

If the user has Magit, the buffer will update when a change is committed or staged.

If the user doesn't have Magit, then nothing changes.  Emacs will create a variable `magit-revert-buffer-hook` that is never run.  No error is produced.

The motivation for this fix is that the git-gutter indicator `[+-=]` would not update after committing a change in Magit.  I think this fixes a common pain point for many users.  

Here's a related stackoverflow question that prompted this tweak.
http://stackoverflow.com/questions/23344540