Fix error when matching languages against heuristics

This issue was reported in #1808 and has some quite big consequences.
This error occurred because [`Heuristic.matches?`](https://github.com/github/linguist/blob/master/lib/linguist/heuristics.rb#L54-L57) doesn't check that the list of languages to match against the heuristic rules is not empty.

I tested it against the [bootstrap repository](https://github.com/twbs/bootstrap).

/cc @bkeepers