Simple script for downloading latest CLI.

This is pretty simple right now and has a number of limitations, but probably "good enough" to start with:

* hard coded for Scala 2.11
* hard coded for CDH5
* assumes the existence of a "latest" file which is currently updated manually until promulgate is updated (https://github.com/ambiata/promulgate/issues/3)
I didn't know about `mktmp`. Handy.