Fix version output in `--help`

--help says the version is 0.0.0, when --version has the real version number.

$ gitlab-runner --help
NAME:
   gitlab-runner - a GitLab Runner

USAGE:
   gitlab-runner [global options] command [command options] [arguments...]
   
VERSION:
   0.0.0
...
$ gitlab-runner --version
Version:      1.2.0
Git revision: 3a4fcd4
Git branch:   HEAD
GO version:   go1.6.2
Built:        Sun, 22 May 2016 20:05:30 +0000
OS/Arch:      darwin/amd64