Make Electron a devDependency

I moved it from devDependencies to dependencies when we added the app
to npm. But now that that's gone, let's move it back.

Functionally, this causes no difference since electron-packager
automatically excludes `electron` and all devDependencies from the
packaged app.

package.json

@@ -24,7 +24,6 @@
     "deep-equal": "^1.0.1",
     "dlnacasts": "^0.1.0",
     "drag-drop": "^2.12.1",
-    "electron": "1.4.2",
     "es6-error": "^3.0.1",
     "fn-getter": "^1.0.0",
     "iso-639-1": "^1.2.1",
@@ -55,6 +54,7 @@
     "buble": "^0.14.0",
     "cross-zip": "^2.0.1",
     "depcheck": "^0.6.4",
+    "electron": "1.4.2",
     "electron-osx-sign": "^0.3.0",
     "electron-packager": "^8.0.0",
     "electron-winstaller": "^2.3.0",