check/fix virtual machine specs

[orbeckst]

As a last resort, people can also use VMs (in vm subdir) but installing VirtualBox and Vagrant comes with its own set of problems. Nevertheless, the VMs are useful for us for testing installation recipes (at least different Linux distros).

I have a suspicion that the VMs that I built do not work anymore because the URLs of the boxes are not valid anymore. It might be sufficient to replace them with "bento"-based boxes from https://atlas.hashicorp.com/bento/ or search for official boxes at https://atlas.hashicorp.com/boxes/search .

The ones that should work/should be tested:

• Debian
• Ubuntu
• CentOS

[orbeckst]

Working again but I didn't have time to make a CentOS one.

I removed the VMs from this repo; they exist both in the MDAnalysisTutorial and the MDAnalysis/mdanalysis repo.