NPM postinstall hooks not run with npm 5.3.0

As tested on two Ubuntu boxes and a Mac:

```bash
$ npm --version
5.3.0
$ npm install -g create-elm-app
# ... runs but doesn't call the postinstall hooks to create the shim
$ asdf global nodejs 8.1.2
$ npm --version
5.2.0
$ npm install -g create-elm-app
# ... runs and creates the shim as expected.
```