Add coq-HoTT as a dependency for the .vo files

Some basic testing suggests that this will cause `make` to auto-re-coq
the .v files whenever you `git submodule update`, i.e., when it's
possible to tell that the version of Coq we're using has changed.
Merge pull request #464 from JasonGross/coq-HoTT-depend

Add coq-HoTT as a dependency for the .vo files