Update the OSX service instruction

This tries to answer and notice most common problems when using Runner as an service on OSX.

We should add to a blog post information about this findings and advise any OSX user to upgrade startup script:

gitlab-runner uninstall
gitlab-runner install
gitlab-runner start

The binary:

• https://gitlab-ci-multi-runner-downloads.s3.amazonaws.com/osx-launch-agents/binaries/gitlab-ci-multi-runner-darwin-amd64

@axil Could you review?

Fixes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/972 https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/217 https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1143 https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/122