Update dependency prettier to v1.15.2

Author: renovate-bot

package-lock.json

@@ -49,7 +49,7 @@
     "jest": "23.6.0",
     "ncp": "2.0.0",
     "node-fetch": "2.3.0",
-    "prettier": "1.13.7",
+    "prettier": "1.15.2",
     "stats-webpack-plugin": "0.7.0",
     "url-join": "4.0.0",
     "webpack": "4.25.1",