fix(package): update prop-types to version 15.6.1

Author: greenkeeper[bot]

package.json

@@ -42,7 +42,7 @@
   "dependencies": {
     "@uirouter/core": "5.0.17",
     "classnames": "2.2.5",
-    "prop-types": "15.6.0"
+    "prop-types": "15.6.1"
   },
   "peerDependencies": {
     "react": "^15.0.0 || ^16.0.0"