Make image_style_quality dependency optional

This is a good idea. Since we are already working on 2.x-dev which improves codebase to play well with contrib ecosystem, we could include this change in there.

@grimreaper,

This MR revealed an issue that composer.json was not up-to-date with the actual dependencies. I have added that for now. This should not be in conflict with this Issue, as in 2.x we can have better management for optional dependencies.

Ok, thanks for the info.

Also composer.json was completed on the fly by drupal.org composer facade from the .info.yml file.

But for documentation/sync purpose that's nice!