Move current version into separate directory

The current Dockerfile builds the latest 0.21.0.  I'm about to add tags
for a handful of previous versions, and to keep things simple, I'm just
going to have a separate directory for each version.
Mesos 0.20.1
Mesos 0.20.0
Mesos 0.19.1
Merge branch 'tags'

* tags:
  Mesos 0.19.1
  Mesos 0.20.0
  Mesos 0.20.1
  Move current version into separate directory