RFC use ROOT binary distribution packages?

[tiborsimko]

Currently, ROOT6 container image is compiled from ROOT sources, which takes about an hour or so, which makes Travis CI to time out. So the tests were swtiched off for the time being.

@lukasheinrich Did you have some particular reason to use Ubuntu and to compile ROOT from the sources? May I switch to say CentOS7 and use pre-compiled ROOT binary distribution packages from https://root.cern.ch/download/?

This would enable us to build images faster, to plug Travis CI, etc. Please let me know.

[dprelipcean]

Also to consider: We could use the latest ubuntu images, as from ubuntu 17 onwards python3 comes by default, since we approach the end of python2.