Removed not needed anymore `replace` function

1. Removed not used in nib anymore `replace` function. This is important, because in Stylus 0.36.0 a proper `replace` function appeared, and if you'd include nib, things would go wrong, 'cause its replace overrides Stylus' replace.

2. Updated Stylus in package.json to `0.37.0`, it's the latest version and there is no reason not to use it :)

3. Removed commented code for old gradient mixin, as we now use a new one.