better docker version check
better docker version check

Fixes #567. Supersedes #1298.

cgvarela/weave,tomwilkie/weave,awh/weave,tomwilkie/weave,narry/weave,bboreham/weave,tomwilkie/weave,narry/weave,paulbellamy/weave,cgvarela/weave,cgvarela/weave,squaremo/weave,beni55/weave,beni55/weave,awanke/weave,n054/weave,beni55/weave,weaveworks/weave,bboreham/weave,aaam/weave,n054/weave,awanke/weave,aaam/weave,tomwilkie/weave,bboreham/weave,paulbellamy/weave,awh/weave,dpw/weave,weaveworks/weave,narry/weave,n054/weave,awanke/weave,paulbellamy/weave,cgvarela/weave,rade/weave,narry/weave,awh/weave,weaveworks/weave,squaremo/weave,philipz/weave,aaam/weave,philipz/weave,aaam/weave,beni55/weave,dpw/weave,rade/weave,n054/weave,awanke/weave,dpw/weave,weaveworks/weave,beni55/weave,awh/weave,rade/weave,narry/weave,n054/weave,tomwilkie/weave,cgvarela/weave,paulbellamy/weave,squaremo/weave,squaremo/weave,philipz/weave,aaam/weave,rade/weave,awanke/weave,philipz/weave,squaremo/weave,dpw/weave,bboreham/weave,dpw/weave,rade/weave,bboreham/weave,philipz/weave,awh/weave