fix: save package in devDependencies

In the latest versions of the CLI `ng-add` packages can be added to `devDependencies` and this package is perfect for such use case since it's only needed for development.

See: angular/angular-cli#15815
(cherry picked from commit 3dfd8d64a6781a4081816a2022a66283c2b7247b)

closes #12
see angular-schule/angular-cli-ghpages#81

src/package.json

@@ -9,6 +9,9 @@
   },
   "schematics": "./collection.json",
   "builders": "./builders.json",
+  "ng-add": {
+    "save": "devDependencies"
+  },
   "repository": {
     "type": "git",
     "url": "git+https://github.com/angular-schule/ngx-deploy-starter.git"