Enable support for cli v7

[bonzofenix]

When trying to use with newer cf release I am getting the following error:

Cloud Foundry API version 2.155.0 requires CLI version 7.0.1. You are currently on version 6.43.0+815ea2f3d.2019-02-20. To upgrade your CLI, please visit: https://github.com/cloudfoundry/cli#downloads

[bonzofenix]

Built this image in the meantime to use the cf cli v7:

https://hub.docker.com/r/bonzofenix/cf-resource

usage:

...

resource_types:
- name: cf
  type: registry-image
  source:
    repository: bonzofenix/cf-resource