Update Go instructions in README
Update Go instructions in README

Go 1.4.x no longer builds this project; Update to install 1.5.x.
hu19891110/gitlab-ci-multi-runner,hu19891110/gitlab-ci-multi-runner,soiff/gitlab-ci-multi-runner,soiff/gitlab-ci-multi-runner,soiff/gitlab-ci-multi-runner,hu19891110/gitlab-ci-multi-runner