Version 2.1
o Update to 0MQ 4.x
o Clean up and reduce size of Docker images
o Begin to tidy up and document Packer.io build of Vagrant boxes
o Fix handling of non-response failure in cpppo.server.enip.client
Further refine and document Packer.io usage
Version 2.2.  Merge branch 'feature-zeromq-4'