Add branch name to the version info
Adds compilation branch name to the version info:
$ gitlab-runner --version
Version: 1.2.0~beta.107.g30703b7
Git revision: 30703b7
Git branch: feature/extend-version-info-with-branch-name
GO version: go1.6.2
Built: Sat, 14 May 2016 15:31:58 -0500
OS/Arch: linux/amd64