Fixes #2124: ignore bad local override revisions if updating the dependency anyway

So this is a spec and a fix for issue #2124.

The change prevents the "The Gemfile lock is pointing to revision ..." error being triggered for a gem if you've called bundler with `bundle update <gem>` listing that same gem. Given the old revision is about to be discarded in that case, it doesn't matter if it points to an orphaned sha.

It's my first PR for the project so let me know if I'm doing anything wrong.