Travis: allow user forks

Authored by thomie.

Description

Travis: allow user forks

Check for each submodule repository url if the user made its own fork.
If so, use that. Otherwise, fall back on the one from github.com/ghc.

As suggested by Richard in this mailinglist discussion:
https://mail.haskell.org/pipermail/ghc-devs/2014-November/007300.html

Documentation is at [wiki:TestingPatches#Travis].

Differential Revision: https://phabricator.haskell.org/D939