Use MaskLexer for *.mask

Hello Team, 

it seems `pygments.rb` was updated, and the `MaskLexer` is included ([mapping](https://github.com/tmm1/pygments.rb/blob/master/vendor/pygments-main/pygments/lexers/_mapping.py#L214)), so this pr changes the lexer. 

Thank you, Alex
@aroben :eyes: Is this cool? Any pointers on where I should look to verify the `tm_scopes`?