Suggestion: Remove 'mathspec' from utwentefont.sty for LuaLaTex compatibility

utwentefont.sty can currently not be built using LuaLaTex because if you want to use mathspec, then at present you need to run XeLaTeX.

I'm attempting to replace mathspec with fontspec and mathastext which are both compatible with XeLaTeX and LuaLaTex, are you keen for a Merge Request that makes a change like this?