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

Closes #3

Author: greenkeeper[bot]

package.json

@@ -8,7 +8,7 @@
     "gulp-if": "^2.0.0",
     "gulp-minify-css": "^1.2.1",
     "gulp-replace": "^0.6.1",
-    "gulp-sass": "^3.1.0",
+    "gulp-sass": "^4.0.1",
     "gulp-uglify": "^3.0.0",
     "js-yaml": "^3.4.3"
   },