Upgrade to React v15.1.0

This version seems to produce some spurious warnings like these:
> Element: `key` is not a prop. Trying to access it will result in
> `undefined` being returned. If you need to access the same value
> within the child component, you should pass it as a different prop.

I'm pretty sure these warnings can be safely ignored and will go away
in the next release (see facebook/react#6880).

app/package.json

@@ -36,8 +36,8 @@
     "mobx-react": "^3.1.0",
     "mobx-react-devtools": "^4.0.5",
     "polymer-ts-decorators": "github:enlight/polymer-ts-decorators",
-    "react": "^0.14.7",
-    "react-dom": "^0.14.7",
+    "react": "^15.1.0",
+    "react-dom": "^15.1.0",
     "react-free-style": "^2.2.0"
   }
 }