Upgrade to lib-sass 3.5.6 with rgba vars support

[jrgcubano]

Really waiting to use the fix from libsass (3.5.6) in reference to rgba and rgb using custom properties aka var.

Thanks in advance

[finnhvman]

I also need support for color: rgb(var(--my-var)); and such as well. People cannot use my library with Sass because of this.

@jrgcubano how is it known that libsass (3.5.6) will fix this? Also, I don't see a 3.5.6 release of libsass.

[finnhvman]

Any movement?
It's been more than two months

[saper]

Closing as duplicate of #2251