It seems that the package.json is broken in the template. See https://github.com/driftyco/ionic-cli/issues/1660#issuecomment-260122096.
To fix, remove “ionic:build” and “ionic:serve” in the “scripts” part and leave it like that:
"scripts": {
"build": "ionic-app-scripts build",
"watch": "ionic-app-scripts watch"
},
It’s commented https://github.com/driftyco/ionic-app-scripts#npm-scripts.