Update prettier to version 1.15.0 (#248)

Author: greenkeeper[bot]

package.json

@@ -38,7 +38,7 @@
     "eslint-config-problems": "1.1.0",
     "nyc": "^13.1.0",
     "postcss-import": "^12.0.0",
-    "prettier": "~1.14.0",
+    "prettier": "~1.15.0",
     "sugarss": "^2.0.0",
     "uuid": "^3.0.1"
   },