Merge branch 'sh-bump-gomod-version' into 'master'

Bump Go version in go.mod to v1.13

See merge request gitlab-org/gitlab-workhorse!552

go.mod

 module gitlab.com/gitlab-org/gitlab-workhorse
 
-go 1.12
+go 1.13
 
 require (
 	github.com/BurntSushi/toml v0.3.1