Modify AutoUpdater behavior on macOS.

The AutoUpdater requires that macOS builds be code signed. That is currently not possible.
As a workaround, the autoupdater on mac will now alert users that an update is available and give the option to download the file directly from GitHub.
Closes #16.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "westeroscraftlauncher",
-  "version": "1.0.6-pre.2",
+  "version": "1.1.0-pre.1",
   "productName": "WesterosCraft Launcher",
   "description": "Modded Minecraft Launcher",
   "author": "Daniel Scalzi (https://github.com/dscalzi/)",
@@ -76,6 +76,7 @@
       "uninstallerIcon": "build/icon.ico"
     },
     "mac": {
+      "target": "dmg",
       "category": "public.app-category.games",
       "icon": "build/icon.icns"
     },