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
This will all become less confusing when we integrate the language grammars we're using on github.com into Linguist. Once that is done, we'll want to make sure we have a grammar for every `Linguist::Language#tm_scope` (some of which are derived from the language name, others which are specified in the yaml file).

A quick search does not turn up a Mask grammar for TextMate, Sublime Text, or Atom, so for now I think we should probably leave this as `source.scss` until we have something better.