Use slick fork to prevent infinite loop

Parsing invalid selectors such as `()` using slick causes infinite loops when the parser does not advance.

This PR switches to using a fork of [slick](https://github.com/atom/slick) that removes this infinite loop.

This makes the `atom.syntax` API much more resilient to invalid input that would cause infinite loops and also better handles grammars that generate scope names that cannot be parsed by slick.