Update reference to scssphp with BC

[sad270]

scssphp has moved. This PR updates the composer recommendation.
This is same PR #892 with BC requested by @stof in this comment

Fix #896

[LukeTowers]

This has been implemented in the 2.0 branch of assetic-php/assetic. Until we tag 2.0, you can pull it in by using "assetic/framework": "dev-2.0/dev" in your composer.json files.

[sad270]

Hi, @LukeTowers thanks for your work, but i need stable project with php 7.1 assetic-php/assetic need php 7.2 or higher

[LukeTowers]

@jaxwilko what are we using that requires PHP 7.2? Can we bump it down to 7.1?

[sad270]

@LukeTowers Maybe it's better to enable the issue tab in assetic-php/assetic repository and create new issue "Bump php down to 7.1 or 7.0" and talk about this in this new issue :)

And add link to this new issue in this PR

Or you can add my PR in your Repository, and release a v1.5.0 and keep a v1 version who fix all issue of this repository without new feature or new php version :)