[magnum]: do not use deprecated Magnum functions

[costashatz]

Update for latest upstream Magnum...

[jbmouret]

shouldn't we merge this to `devel' and keep the master to a specific version of Magnum?

Merging this will break all the installations/containers that did not update Magnum yet, I guess...

[costashatz]

We need to first have a devel branch.. ;)

I think that I like more the following: master is development, other branches or tags for stable things. I can create one for your needs. What do you think?

[jbmouret]

Yes, this works for me too, but we need to keep a "release x" branch in that case :)

[costashatz]

Good! I'll first make a PR to remove (the dead now) travis and use GitHub actions for CI. Then I'll create separate release branches..

[costashatz]

@jbmouret I'll merge this and let me know if things will break for your dockers/setups. Of course the easiest solution is just to use latest Corrade/Magnum etc, but once we make the 1.0.0 release we'll provide installation instructions for specific commits of Corrade/Magnum etc for easy portability and avoid issues.