Bump prop-types due to duplicate package warning during tests

Author: darrenscerri

packages/react-scripts/fixtures/kitchensink/.template.dependencies.json

@@ -7,7 +7,7 @@
     "jsdom": "9.8.3",
     "mocha": "3.2.0",
     "normalize.css": "7.0.0",
-    "prop-types": "15.5.6",
+    "prop-types": "^15.6.0",
     "test-integrity": "1.0.0"
   }
 }