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[1].  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.

[1]: https://groups.google.com/a/cloudfoundry.org/d/msg/vcap-dev/8caLIfCusvU/aULrvbJ21AMJ