Both SPDY an gzip_static are needed for upcoming GitLab SR:

  - GitLab uses SPDY in its https nginx configuration, and
  - prepares compiled assets in pre-gzipped form on filesystem

both modules are off by default and need to be explicitly enabled with
corresponding directives, so this should not affect already used nginx
configurations.

/cc @kazuhiko, @jerome, @gabriel
/reviewed-by @rafael  (on nexedi/slapos!30)