The latest Unity Editor for linux (5.3 and above) will require Mono 4.0.
When installing in Ubuntu the .deb package will pull all the dependencies correctly, however in RHEL 7 (or derivates) the dependencies will need a bit of extra love.

After adding the mono repo (instructions here)
You will need to install mono-devel and mono-complete, but you will be met with 20+ screens of packages requirements fails:

Some how, yum is not able to make the link between mono-devel, mono-complete, and mono-core.
And of course, RHEL will only have mono-core-2.0.

You will need to manually remove all mono packages in your system, before trying to install mono from its repo.

A quick and dirty way of doing it:

You will now have mono-core 4.0, ready for your Unity playground.