cloudfoundry-cli 6.1.2

This change updates the formula to install cloudfoundry-cli `v6.1.2`.

Significant changes were made to this formula to work with the new dependency strategy [used by the project](https://groups.google.com/a/cloudfoundry.org/d/msg/vcap-dev/8caLIfCusvU/aULrvbJ21AMJ).  As a side-effect of the new dependency strategy, the source code can now be downloaded via a tarball rather than a Git clone.  An unfortunate other side-effect is that there is now a build-time dependency on Mercurial.