chore(package): update gulp-clean-css to version 3.7.0

Closes #82

Author: greenkeeper[bot]

package.json

@@ -61,7 +61,7 @@
     "font-awesome": "4.7.0",
     "gulp": "3.9.1",
     "gulp-babel": "6.1.2",
-    "gulp-clean-css": "3.4.1",
+    "gulp-clean-css": "3.7.0",
     "gulp-nodemon": "2.2.1",
     "gulp-postcss": "7.0.0",
     "gulp-rename": "1.2.2",