Bundle mac app as dmg instead of 7z.

package.json

@@ -201,7 +201,7 @@
       "category": "public.app-category.social-networking",
       "icon": "build/icons/mac/icon.icns",
       "target": [
-        "7z"
+        "dmg"
       ],
       "bundleVersion": "1",
       "hardenedRuntime": true,
@@ -209,6 +209,9 @@
       "entitlements": "build/entitlements.mac.plist",
       "entitlementsInherit": "build/entitlements.mac.plist"
     },
+    "dmg": {
+      "sign": false
+    },
     "win": {
       "asarUnpack": "node_modules/spellchecker/vendor/hunspell_dictionaries",
       "artifactName": "${name}-win-${version}.${ext}",