Make bundler display post_install_message's for gems installed via :git or :path

This patch makes bundler display post install messages for gems installed via :git or :path.  

Awhile back I forked a gem and added some functionality.  A number of people starting using my :git repo.  My changes are now in the main gem and I'd like people to stop using my repo.  I'd like to notify them about it without affecting the code.  I could update the README, but that assumes they'd go to github and read it.

This change would allow me to add a post_install_message to my version of the gemspec and the next time they update, they'd get notified.

For :git, it will only display if the repo is updated.  For :path it will always show.  I didn't see an obvious way of indicating that :path gems haven't changed.  But I imagine that's a small edge case.