reorg tests into unit and functional

unit tests run first, then functional tests.
fail early on version mismatch between Docker and docker-api gem

* Show docker version at beginning of wercker job
* Check if docker-api gem matches docker version before we
  build the docker image
restrict docker-api gem to be compatible with wercker image

wercker currently uses docker version 0.11.1, which
is incompatible with the newly-release docker-api gem 1.13.0.
launch one user devenv before test suite for testing

If we start a container before the test suite, then
individual rspec examples can just ssh into the container
instead of starting and stopping containers repeatedly.