Running `npm rebuild node-sass` as we instruct people, has been
noop for a while. I'm not entirely sure what has changed.

Debugging as reveal that running `rebuild` now just triggers the
`install` and `postinstall` scripts. The `build` script is no
longer run. This suggests something has changed in newer versions
of npm.

Since the `--force` flag is required to rebuild the binary I've
update the `build` script to take into account if `--force` was
passed to npm.