set appdmg package optional

package.json

@@ -33,7 +33,6 @@
   "homepage": "https://github.com/Rokt33r/codexen-app#readme",
   "dependencies": {
     "@rokt33r/node-ipc": "^5.0.4",
-    "appdmg": "^0.3.5",
     "devicon": "^2.0.0",
     "electron-gh-releases": "^2.0.2",
     "font-awesome": "^4.3.0",
@@ -74,6 +73,9 @@
     "webpack": "^1.12.2",
     "webpack-dev-server": "^1.12.0"
   },
+  "optionalDependencies": {
+    "appdmg": "^0.3.5"
+  },
   "standard": {
     "ignore": [],
     "globals": [