Stop using `Bundler.settings.with`, because it will be removed from
Bundler 1.6.

Also, as described in #7653, we could use `Bundler.settings[:with]`, but
that would be internal API again, so it probably breaks again in the
future. That's why I added a `BundlerHelper` module to parse the
required optional group from our config files, without the use of any
internal Bundler API.

Fixes #7653