Added prop-types explicitly to package.json

Author: brendanmh

package.json

@@ -46,6 +46,7 @@
     "karma-webpack": "~1.7.0",
     "mocha": "~2.4.5",
     "phantomjs-prebuilt": "~2.1.7",
+    "prop-types": "~15.5.8",
     "react": "^15.0.1",
     "react-addons-test-utils": "^15.0.1",
     "react-dom": "^15.0.1",