nginx: use a compression level of 5 in recommended configuration

While there is little gain of space to use a compression level of 9,
the CPU usage is significant. Many experiments point to use something
between 4 and 6. For example:

 - https://mjanja.ch/2015/03/finding-the-nginx-gzip_comp_level-sweet-spot/
 - https://github.com/h5bp/server-configs-nginx/blob/3bda5b93edba147d51760e900c2079828a7dc274/nginx.conf#L93