Add support for ref: in gem.deps.rb.lock for git

This support was omitted.  The ref: doesn't seem to do anything useful
in bundler (its only fetched for an equality comparison) so it is
ignored here.

Bug #822
Add support for git branch: in gem.deps.rb.lock

This support was omitted.  Like ref: this appears to be ignored (see
@56aa22c).

Bug #822
Add support for git tag: in gem.deps.rb.lock

This support was omitted.  Like ref: this appears to be ignored (see
@56aa22c).

Bug #822