Change: Only update cf-upgrade when needed

The cf-upgrade binary was broken on 3.6.0 and 3.6.1. The policy to
download a working cf-upgrade binary from the hub is unnecessary when
running newer versions. This change suppresses a promise not kept when
performing upgrades in some cases and reduces unnecessary work.
Merge pull request #383 from nickanderson/fix_cfupgradefix

Change: Only update cf-upgrade when needed