chore: pin prop-types to 15.6.2

Author: greenkeeper[bot]

package.json

@@ -43,7 +43,7 @@
   },
   "dependencies": {
     "lodash.isequal": "^4.5.0",
-    "prop-types": "^15.6.1"
+    "prop-types": "15.6.2"
   },
   "devDependencies": {
     "@babel/cli": "^7.1.0",