debian/control: Update the repository to github.com.

I keep my development of the package (in particular, the master branch) at
GitHub and I usually try to have my changes there, since I am not always
with my SSH/GPG keys at hand.

For the benefit of the co-maintainers, I keep a backup copy of the changes
at git.debian.org, where, due to said keys not being available at all times,
I can't push my changes, which leads, sometimes to my GitHub repository to
be ahead of the Debian mirror.

I prefer that, if possible, when working with the master branch:

* People send me pull requests there
* If they are not comfortable with the use of GitHub for any kind of reason
  (political, ideological etc.), that they send me patches via email (git
  send-email is best).

Signed-off-by: Rogério Theodoro de Brito <rbrito@ime.usp.br>