Add gitlab library to vendor path.

vendor/github.com/xanzy/go-gitlab/CHANGELOG.md

vendor/github.com/xanzy/go-gitlab/LICENSE

vendor/github.com/xanzy/go-gitlab/README.md

vendor/github.com/xanzy/go-gitlab/branches.go

vendor/github.com/xanzy/go-gitlab/build_variables.go

vendor/github.com/xanzy/go-gitlab/builds.go

vendor/github.com/xanzy/go-gitlab/commits.go

vendor/github.com/xanzy/go-gitlab/deploy_keys.go

vendor/github.com/xanzy/go-gitlab/events.go

vendor/github.com/xanzy/go-gitlab/gitlab.go

vendor/github.com/xanzy/go-gitlab/groups.go

vendor/github.com/xanzy/go-gitlab/issues.go

vendor/github.com/xanzy/go-gitlab/labels.go

vendor/github.com/xanzy/go-gitlab/merge_requests.go

vendor/github.com/xanzy/go-gitlab/milestones.go

vendor/github.com/xanzy/go-gitlab/namespaces.go

vendor/github.com/xanzy/go-gitlab/notes.go

vendor/github.com/xanzy/go-gitlab/project_snippets.go

vendor/github.com/xanzy/go-gitlab/projects.go

vendor/github.com/xanzy/go-gitlab/repositories.go