Explicitly set Sass compiler for gulp-sass (#7948)

As per the gulp-sass docs [1] we should be setting the `sass.compiler` property
for forwards-compatibility "in case the default ever changes."

  [1]:https://github.com/dlmanning/gulp-sass/tree/v4.0.2#basic-usage

gulpfile.js

@@ -29,6 +29,8 @@ const { makeStringTransform } = require('browserify-transform-tools')
 
 const packageJSON = require('./package.json')
 
+sass.compiler = require('node-sass')
+
 const dependencies = Object.keys(packageJSON && packageJSON.dependencies || {})
 const materialUIDependencies = ['@material-ui/core']
 const reactDepenendencies = dependencies.filter(dep => dep.match(/react/))