update postcss-import to v4

By updating postcss-import to version 4, it is possible to `@import` files in a manner similar to how [rework-npm](https://github.com/reworkcss/rework-npm) does.

Basically, instead of needing to do:
```
@import 'normalize.css/normalize.css';
```

It is possible to do:
```
@import 'normalize.css';
```

More detailed information is available [here](https://github.com/postcss/postcss-import/issues/7)

Cheers!