Update proxy URL to use PUMA defaults (0.0.0.0:9292)

as apache's mod_proxy doesn't support sockets [1].
This obviously implies that `config/puma.rb` is configured accordingly (and assumes gitlab v5.1):

    # Bind the server to “url”. “tcp://”, “unix://” and “ssl://” are the only
    # accepted protocols.
    #
    # The default is “tcp://0.0.0.0:9292”.
    #
    bind 'tcp://0.0.0.0:9292'
    #bind "unix://#{application_path}/tmp/sockets/gitlab.socket"

[1] http://httpd.apache.org/docs/2.2/mod/mod_proxy.html