warning :: 5.4.0 in the stable branch

Mono 5.4 is on the Stable branch now.

Mono 5.4.0.201 has not been tested with Xamarin Android,
but 5.4.0.199 have very serious issues with Xamarin Android,
so probably 5.4.0.201 will not work for Xamarin Android / Xamarin.Forms today.
Anyway it should be verified (maybe I will try it a bit later).

Please use this:
http://www.mono-project.com/docs/getting-started/install/linux/#accessing-older-releases

My settings:
deb http://download.mono-project.com/repo/debian stretch/snapshots/5.2.0.224 main