Upgrade to Electron v1.2.1

There was a bug in the V8 version in Electron v1.1.0 that made debugging
the main process rather impractical because the `node-inspector` UI
often blew up with an `Internal Error: Invalid Acccess` error when a
breakpoint was hit in the main process.

Relevant `node-inspector` issue:
node-inspector/node-inspector#864

package.json

@@ -36,8 +36,8 @@
     "babel-cli": "^6.8.0",
     "babel-plugin-transform-polymer-base": "github:enlight/babel-plugin-transform-polymer-base",
     "crisper": "^2.0.2",
-    "electron-prebuilt": "^1.1.0",
-    "electron-rebuild": "^1.1.4",
+    "electron-prebuilt": "1.2.1",
+    "electron-rebuild": "1.1.5",
     "grunt": "^0.4.5",
     "grunt-contrib-jshint": "^0.11.3",
     "grunt-sync": "^0.4.1",