feat(generators/app/templates): transition to use prepublishOnly in…

… favor of `prepublish`

This is an npm@>=5 feature so also add an `engines` hint for it.

As per this `npm@5` warning:

> ```
> npm WARN prepublish-on-install As of npm@5, `prepublish` scripts are deprecated.
> npm WARN prepublish-on-install Use `prepare` for build steps and `prepublishOnly` for upload-only.
> npm WARN prepublish-on-install See the deprecation note in `npm help scripts` for more information.
> ```

generators/app/templates/@README.md

@@ -41,7 +41,6 @@ This will:
   * _\[... Ensure you have local ... and ... branches tracking their respective remote counterparts;\]_
   * _\[... Set up the ... [branching model](#branching-model);\]_
   * Install all required dependencies;
-  * The latter command will also invoke `grunt` (no args) for you, creating a production build in `./dist` (plus artifacts);
 
 
 ### Build

generators/app/templates/@package.json

@@ -35,6 +35,9 @@
   "directories": {
     "doc": "dist/doc"
   },
+  "engines": {
+    "npm": ">=5"
+  },
   "files": [
     "AUTHORS",
     "CHANGELOG.md",
@@ -55,7 +58,7 @@
     "dist": "true",
     "prebuild": "rm -rf \"${npm_package_config_dist}\"",
     "predist": "npm run build",
-    "prepublish": "npm run dist",
+    "prepublishOnly": "npm run dist",
     "release": "standard-version",
     "test": "command -v grunt > /dev/null || { echo >&2 'It appears that \"grunt\" is not installed.  Consider running \"[sudo ]npm install -g grunt-cli\" first.'; exit ; } && grunt test"
   },