Support Go 1.12

Since today a new Golang version has been released, 1.12. In itself its always a good to keep current with Golang versions. And Go 1.12 has additional benefits for the Gitaly team.

Go 1.12 marks the second release for go mod. go mod allows for dependency management without depending on $GOPATH. This is especially nice for both contributors and GitLab CI which need special instructions to get started.

Possible downsides I can think of right now is that govendor generates our notice, and a replacement for that I've not found yet.

The introduction consists of multiple steps:

• Update the GitLab Build Images
• Introduce Go 1.12 in the CI matrix

Introducing go mod is not a part of this issue, but will be handled by #1514 (closed).