chore(deps): update dependency prop-types to v15.8.1

Author: renovate-bot

package-lock.json

@@ -74,7 +74,7 @@
     "node-sass": "6.0.1",
     "npm-run-all": "4.1.5",
     "nyc": "15.1.0",
-    "prop-types": "15.7.2",
+    "prop-types": "15.8.1",
     "react": "16.10.1",
     "react-addons-test-utils": "15.6.2",
     "react-dom": "16.2.1",