Add batsim to SimGrid's nightly CI

As #37 (closed) seems on its way to be fixed, we should take some time to include Batsim to SimGrid continuous integration suite.

This would allow SimGrid developers to detect more easily when they break things, as Batsim uses many SimGrid features.

This would also be very beneficial for Batsim.

This would limit the appearance of deep breaks (e.g., #37 (closed)), as we would seem them as soon as they appear (and can request change in SimGrid at this time or do it ourselves).
This would help keeping up-to-date with SimGrid, as breaks would be detected sooner.
Knowing whether Batsim works with up-to-date SimGrid would also be beneficial to end users, avoiding the version hell we had with SimGrid's clone (for batsim-2.0.0).

It would be interesting to know (at least):

Whether the latest Batsim release works with the latest SimGrid commit.
Whether the latest Batsim version (master branch) works with latest SimGrid commit.

Many ways to do it. The more obvious one is to repeat what has been done for StarPU. This has been done on Jenkins (https://ci.inria.fr/simgrid/).

As SimGrid uses many other CI systems, we could also provide something else.

SimGrid dev repo is now hosted on framagit, which should allow us to easily use our Nix recipes on Gitlab CI.
We may hack something on Travis (GitHub)
We may also set up an Hydra infrastructure.

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