fix(package): update gulp-sass to version 4.0.1

Closes #90

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
     "gulp": "^3.9.0",
     "gulp-concat": "^2.6.0",
     "gulp-postcss": "^7.0.0",
-    "gulp-sass": "^3.1.0",
+    "gulp-sass": "^4.0.1",
     "merge-stream": "^1.0.0",
     "metaquery": "^1.2.5",
     "node-sass": "^4.5.3",