A week or two back, the master git branch for MonoDevelop revved to version 5.1. Unfortunately, it doesn’t look like Linux rates very high on the Xamarin guys list nowadays, and there’s no add-in repo for it.
I’ve tried building the current MonoGame and MonoGameContent addins from source and putting them in the local directory, but each has different problems with MD5.1 and are unusable in their current state. Hopefully someone more familiar with the MD add-in system will figure it out soon. I’m going back to using the Utopic MD4.2.3 package for now.