Switch back to Squirrel installer

which it turns out is by far the lesser of two evils.

 * Auto-update works with a proxy
 * The update process is reasonably atomic & faster, rather than
   running the uninstaller then the installer, leaving you with a
   broken install if you shut down your machine at the wrong time
 * Gets the update URL the same way as on mac, rather than baking
   it into the app at build time from package.json. We don't want
   it in package.json because only our builds want our update URL.

package.json

@@ -58,7 +58,6 @@
     "browser-request": "^0.3.3",
     "classnames": "^2.1.2",
     "draft-js": "^0.8.1",
-    "electron-auto-updater": "^0.6.2",
     "extract-text-webpack-plugin": "^0.9.1",
     "favico.js": "^0.3.10",
     "filesize": "^3.1.2",
@@ -137,13 +136,15 @@
     "files": [
       "electron/src/**",
       "electron/img/**",
-      "node_modules/electron-auto-updater/**",
       "webapp/**",
       "package.json"
     ],
     "linux": {
       "target": "deb",
       "maintainer": "support@riot.im"
+    },
+    "win": {
+        "target": "squirrel"
     }
   },
   "directories": {