Golang: update version to 1.12.13
Update the used GO version to the latest 1.12.x minor
1.12.13 as of 2019/10/31
See https://golang.org/doc/devel/release.html#go1.12.minor
Closes #20
Relates to https://gitlab.com/gitlab-org/gitlab/issues/30903
Edited by Jason Plum