Use custom versions of electron-squirrel-startup and electron-winstaller

  * Use forked version of two Windows installation modules
  * Do not re-create shortcuts on the desktop on auto-update

Auditors: @bbondy, @alexwykoff

Test plan:

  This change can only be tested via an auto-update

  Scenario 1

  i)   Install older version of browser
  ii)  Remove shortcut from desktop
  iii) Do an auto-update
  iV)  Ensure the shortcut was not created

  Scenario 2

  i)   Install older version of browser
  ii)  Do NOT remove shortcut from desktop
  iii) Do an auto-update
  iv)  Ensure the shortcut opens the most recent version of the browser

Fixes: #1181

package.json

@@ -80,7 +80,7 @@
     "async": "^2.0.1",
     "electron-localshortcut": "^0.6.0",
     "electron-prebuilt": "brave/electron-prebuilt",
-    "electron-squirrel-startup": "^1.0.0",
+    "electron-squirrel-startup": "brave/electron-squirrel-startup",
     "file-loader": "^0.8.5",
     "font-awesome": "^4.5.0",
     "font-awesome-webpack": "0.0.4",
@@ -127,7 +127,7 @@
     "electron-builder": "^2.3.1",
     "electron-packager": "brave/electron-packager",
     "electron-rebuild": "^1.1.1",
-    "electron-winstaller": "^2.3.0",
+    "electron-winstaller": "brave/windows-installer",
     "empty-port": "0.0.2",
     "flow-bin": "^0.22.1",
     "gulp": "^3.9.0",