Add gitaly server dependency
In GitLab 9.0.0 we want to turn on Gitaly in Omnibus installations. Gitaly is not a required component yet for source installations because we want to iterate on the configuration and architecture some more. Omnibus installations will have the option to completely disable Gitaly if they want to or if necessary. The new GITALY_SERVER_VERSION file which version of the Gitaly server application is needed by the current GitLab version.