Limit max heap to ENV['MEMORY_LIMIT'] - 48M

Yes, 48M is completely arbitrary.

This sets ENV['MONO_GC_PARAMS']=="major=marksweep-par,max-heap-size=ENV['MEMORY_LIMIT']-48M"`

See http://www.mono-project.com/Release_Notes_Mono_2.8#Configuration for details on `MONO_GC_PARAMS` options