Add support for Drush 9 and Drupal 8.4

[pfrenssen]

Drupal 8.4.x works great with Drush 9 dependency wise. For people with a lot of dependencies it might be easier to resolve to a working set of dependencies if we also allow Drush 9 as an option.

[webflo]

But it does not work the other way around. This change would install drush 9 for drupal core 8.3.x

[webflo]

I works because we have prefer-stable enabled.

[dillix]

@webflo now we have stable 8.4 core.

[Axolotlke]

In composer.json there is still a refererence to symfony/css-selector 2.x.
This should also be made compatible with the 3.x components required by Drupal 8.4:
symfony/css-selector (~2.8|~3.0)

[pfrenssen]

OK thanks for the report. I will update the CSS selector.

By the way Drush 8 is now also compatible with Drupal 8.4, so we can declare support for both.

[pfrenssen]

We already had support for Drush 8. Updated the CSS selector requirement and also bumped the minimum Drush 9 version to the latest beta. I think this will probably be good to go if this comes back green.

[pfrenssen]

Merging this in, 8.4.0 has been out for a few weeks already.