Loosen stylus dependency to support 0.38+

Stylus 0.38 is out, but PRs like #47, #48 and #50 are getting old.  Let's let package consumers opt in to new versions of stylus as soon (or late) as they like.  Grunt-browserify [takes this strategy](https://github.com/jmreidy/grunt-browserify/blob/master/package.json#L44).