Make Drupdate compatible with Gitlab

[guillaumev]

Drupdate is currently compatible only with Github. Let's make it compatible with Gitlab as well.

To test, create a public repository on gitlab with a Drupal 7 platform and some outdated modules, and make sure Drupdate can provide a Pull Request for these modules.

[MartinMaritim]

@guillaumev I'll start working on this today.