Support `next` as npm tag in `--scripts-version`

Created by: miraage

Not sure if it is a bug or not (might be intented behavior). Instead of writing ... --scripts-version=2.0.0-next.66cc7a90 we would write ... -scripts-version=next.

Currently it installs zeit/next.js.

// EDIT

Possible solution might touch a single line:

- if (validSemver) {
+ if (validSemver || version === 'next') {

Not sure how "breaking" this change might be.