Sass build fails on node-sass ^3.7

If I try to use dops-components in a project where `node-sass` is of version 3.7 and higher I get the following error message on build:

```
Message:
Module build failed:
%dashicon {
^
You may not @extend an outer selector from within @media.
You may only @extend selectors within the same directive.
From "@extend %dashicon" on line 34 of stdin

in /jetpack/node_modules/@automattic/dops-components/client/scss/extends.scss (line 15, column 1)

```

I was wondering if there's an easy fix for this? This is going to be a blocker if we decide to move to Node 6.x. cc @jeffgolenski @MichaelArestad