Launch script fails on Docker version check in D4M+Kubernetes beta

[pidster]

What you expected to happen?

Weave Net to launch on Docker for Mac

What happened?

Launch failed with the following error: ERROR: Unable to parse docker version

How to reproduce it?

1. Install a Docker for Mac beta that contains Kubernetes
2. Type weave launch.

Anything else we need to know?

++ docker -v
++ sed -n -e 's|^Docker version \([0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\).*|\1|p'
+ DOCKER_VERSION=
+ '[' -z '' ']'
+ echo 'ERROR: Unable to parse docker version'```

## Versions:
<!-- Please paste in the output of these commands; 'kubectl' only if using Kubernetes -->

$ weave version

+ '[' -n '' ']'
+ SCRIPT_VERSION=2.1.3
+ IMAGE_VERSION=latest
+ '[' 2.1.3 = unreleased ']'
+ IMAGE_VERSION=2.1.3
+ IMAGE_VERSION=2.1.3
+ MIN_DOCKER_VERSION=1.10.0
+ DOCKERHUB_USER=weaveworks
+ BASE_EXEC_IMAGE=weaveworks/weaveexec
+ EXEC_IMAGE=weaveworks/weaveexec:2.1.3
+ WEAVEDB_IMAGE=weaveworks/weavedb:latest
+ BASE_IMAGE=weaveworks/weave
+ IMAGE=weaveworks/weave:2.1.3
++ cut -s -d: -f1
++ echo ''
+ PROXY_HOST=
+ PROXY_HOST=127.0.0.1
+ DOCKER_CLIENT_HOST=
+ IP_REGEXP='[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}'
+ CIDR_REGEXP='[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}/[0-9]{1,2}'
+ '[' version = --local ']'
+ case "$1" in
+ '[' -z '' ']'
+ echo 'weave script 2.1.3'
weave script 2.1.3
+ '[' -z '' ']'
+ check_docker_version
++ docker -v
++ sed -n -e 's|^Docker version \([0-9][0-9]*\.[0-9][0-9]*\.[0-9][0-9]*\).*|\1|p'
+ DOCKER_VERSION=
+ '[' -z '' ']'
+ echo 'ERROR: Unable to parse docker version'
ERROR: Unable to parse docker version
+ exit 1

$ docker version

Client:
 Version:	17.12-kube_beta
 API version:	1.35
 Go version:	go1.9.2
 Git commit:	ca0c9dbcb219048a1a61fbf82a2e69f1b9795023
 Built:	Fri Dec 15 10:20:47 2017
 OS/Arch:	darwin/amd64
 Orchestrator:	kubernetes

Server:
 Engine:
  Version:	17.12.0-ce-rc3
  API version:	1.35 (minimum version 1.12)
  Go version:	go1.9.2
  Git commit:	80c8033
  Built:	Thu Dec 14 00:45:43 2017
  OS/Arch:	linux/amd64
  Experimental:	true

$ uname -a

Darwin tachyon.local 17.2.0 Darwin Kernel Version 17.2.0: Fri Sep 29 18:27:05 PDT 2017; root:xnu-4570.20.62~3/RELEASE_X86_64 x86_64

$ kubectl version

Client Version: version.Info{Major:"1", Minor:"8", GitVersion:"v1.8.2", GitCommit:"bdaeafa71f6c7c04636251031f93464384d54963", GitTreeState:"clean", BuildDate:"2017-10-24T19:48:57Z", GoVersion:"go1.8.3", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"8", GitVersion:"v1.8.2", GitCommit:"bdaeafa71f6c7c04636251031f93464384d54963", GitTreeState:"clean", BuildDate:"2017-10-24T19:38:10Z", GoVersion:"go1.8.3", Compiler:"gc", Platform:"linux/amd64"}