Incompatibility with mathastext and XeLaTeX

[michaelroland]

See michaelroland/jku-templates-presentation-latex#12

Due to the latest updates to mathastext, the template no longer compiles and fails with

! Extended mathchar used as mathchar (4203026).
<to be read again> 
                   }
l.1241 ...letters\string-\mathbin\mst@varfam@minus
                                                  
A mathchar number must be between 0 and "7FFF.

Also see 96ca93f

[michaelroland]

Issue has been acknowledged by mathastext author via personal e-mail. Proposal was to stick with everymath for now. A fix for mathastext will be provided before removal of the deprecated everymath option.

For now, this is resolved by #21