Remove references to V3 internal API
In the main Rails app we use exactly the same code for
/api/v3/internal and /api/v4/internal. This means that we can move gitlab-shell to V4 in preparation
for sunsetting V3.
Note to reviewer
I didn't refresh the VCR cassettes, I only manually edited them. The reasons are explained in #90 (closed). I think that we should refresh them in this merge request.
I double checked that V4 works using my local GitLab instance (by pushing a file edit).