Update chroma for libsass 3.5+.

[frjo]

When using lib sass 3.5.0+ you get this error with Chroma:

"DEPRECATION WARNING: Passing a string to call() is deprecated and will be illegal in Sass 4.0. Use call(get-function("lighten")) instead."

This can be fixed by updating the internal _safe-call() function like this:

@function _safe-call($function, $parameters...) {
  @if $function == rgba {
    @return rgba(nth($parameters, 1), nth($parameters, 2));
  }
  @return call(get-function($function), $parameters...);
}

But I found a number of @TODO items about reming workarounds for old lib sass version so I acted on them.

The gh-pages is not updated and I'm sure some other things needs work as well but it's a start.