Bump version to 18.09.0-dev
Bump version to 18.09.0-dev

Releases will be supported for a longer term, and
will be done on a 6-month(ish) cycle.

https://blog.docker.com/2018/07/extending-support-cycle-docker-community-edition/

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

docker/cli,cpuguy83/cli,thaJeztah/cli,thaJeztah/cli,docker/cli,cpuguy83/cli