cli: hosts: print docker x.yy.zzz version only

The cli currently prints something like:

DockerVersion{apiVersion=1.12, arch=amd64, gitCommit=688b5cf-dirty, 
goVersion=go1.2.1, kernelVersion=3.13.0-24-generic, os=linux, 
version=1.0.0} (1.12)

We want something like: 1.0.0 (1.12)
Merge pull request #34 from spotify/dano/cli-fix-hosts-docker-version

cli: hosts: print docker x.yy.zzz version only