Add fly

[chendrix]

We should be able to download fly from our concourse

https://cli.ci.cf-app.com/api/v1/cli?arch=amd64&platform=darwin

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/119205267

The labels on this github issue will be updated when the story is started.