Fix handling of git gem deps in lockfile

Unversioned gem dependencies in the GIT section of a lockfile were
parsed but not tested.  These did not work as missing
GitSpecification#add_dependency was missing.

Now the missing method is implemented.

Fixes #940
Refactor adding versioned GIT dependencies

Now both versioned and unversioned GIT dependencies in a lockfile use
GitSpecification#add_dependency.

This follows @8a3bd00