This package no longer exists so trying to install it broke our build.
It has been renamed docker-ce.

I apologize in advance for the rather long PR, but unfortunately there
is not an easy way to break this up into smaller chunks. This separates
the agent configuration into smaller, more consumable pieces just like
the job specification.

This reverts commit f5532978.

This reverts commit c7726ff5.

- Introduces a travis target in Makefile to update docker and
  install rkt

Uses up one less Travis builder per commit, makes for faster throughput.