Fix docker version check bug for vFile

[lipingxue]

Fixed #2024 .
Test:
With this fix, can install vFile successfully with docker 17.06.0-ce.

root@sc-rdops-vm18-dhcp-57-89:~# docker version
Client:
 Version:      17.06.0-ce
 API version:  1.30
 Go version:   go1.8.3
 Git commit:   02c1d87
 Built:        Fri Jun 23 21:23:31 2017
 OS/Arch:      linux/amd64

Server:
 Version:      17.06.0-ce
 API version:  1.30 (minimum version 1.12)
 Go version:   go1.8.3
 Git commit:   02c1d87
 Built:        Fri Jun 23 21:19:04 2017
 OS/Arch:      linux/amd64
 Experimental: false

root@sc-rdops-vm18-dhcp-57-89:~# docker plugin install --grant-all-permissions --alias vfile lipingxue/vfile:vfile-version-check VFILE_TIMEOUT_IN_SECOND=300
vfile-version-check: Pulling from lipingxue/vfile
2dc10eb4c96b: Download complete 
Digest: sha256:3806a320dcdc2501693fd07c84d805bbe67be0e99f776efbf5ccb787028135a7
Status: Downloaded newer image for lipingxue/vfile:vfile-version-check
Installed plugin lipingxue/vfile:vfile-version-check
root@sc-rdops-vm18-dhcp-57-89:~# 
root@sc-rdops-vm18-dhcp-57-89:~# docker plugin ls
ID                  NAME                DESCRIPTION                           ENABLED
a7a626d7e96a        vfile:latest        VMWare vFile Docker Volume plugin     true
cdfdd62016cd        vsphere:latest      VMWare vSphere Docker Volume plugin   true
root@sc-rdops-vm18-dhcp-57-89:~#