Update prettier to version 1.18.0 (#398)

Author: greenkeeper[bot]

package.json

@@ -33,7 +33,7 @@
     "eslint-plugin-import": "^2.17.1",
     "eslint-plugin-prettier": "^3.0.0",
     "postcss-scss": "^2.0.0",
-    "prettier": "~1.17.0",
+    "prettier": "~1.18.0",
     "sugarss": "^2.0.0"
   },
   "scripts": {