Update debian/watch for GitHub switch

The Coq developers now distribute Coq through GitHub rather than via a
direct download from INRIA. Update debian/watch to point to GitHub.
Tweak the version regex slightly to account for Coq using capital V in
their version numbers (e.g., V8.7.2). Also tweak the regex to ignore
any release with a + in it; those releases are preproduction releases.