9.0.0: Gitlab V4 API compatibility

Changelog

• Gitlab V4 API compatibility!
• Switch from kriswallsmith/buzz to php-http/httplug.
• Use of OptionsResolver to handler collection filters. Many missing filters have been added.
• For other changes, you can see the full changelog.

Upgrading

As this library now depends on php-http/httplug, you have to choose a client. You can pick one from this list or use php-http/guzzle6-adapter which is the most popular. You can also stick with kriswallsmith/buzz by requiring php-http/buzz-adapter.
If you are not familiar with HTTPlug, you can read HTTPlug for library users.

All breaking changes are documented in UPGRADE.md.