--watch not working?

The watch flag doesn't seem to work for me.

To reproduce in empty dir:

```
yarn init -y
yarn add @zeit/ncc
echo "console.log('test');" >> index.js
ncc build index.js # works
ncc build index.js --watch # fails without error
```

Environment:

```
$ node --version
v11.13.0
$ ncc version
0.17.2
```