Fix diffing client to fetch parsoid version from the server.

* The client code was copied from the Parsoid codebase and over
  there, it was okay to look at git version by looking at the
  git repo on disk. However, in this case, we are interested in
  the version of Parsoid we are testing, not the version of the
  visual diffing code. I suppose, strictly speaking, we want to
  use a combination of both .. but ignoring that for now.