Build workhorse with go 1.5.x instead of 1.7.x

We recently bumped the go version to 1.7.4, but have run into issues with http 1.0 connections and nginx-workhorse-shell and are going to switch back to 1.5.4

Go 1.6.x introduced a Host header for http 1.1 connections, and we have nginx upgrading http 1.0 connections to 1.1, but they fail to include the host header, which currently breaks our communication to gitlab shell.

See: https://gitlab.com/gitlab-com/infrastructure/issues/904 for more info