roles/nginx: Reduce gzip_comp_level from 6 to 5

After performing some tests[0] again I see that level 5 produces
the most acceptable results for my site currently. Others[1] are
using the same, so I'll gladly accept a little less CPU load for
a tradeoff of a few bits of file size.

[0] https://mjanja.ch/2015/03/finding-the-nginx-gzip_comp_level-sweet-spot/
[1] https://github.com/h5bp/server-configs-nginx/blob/master/nginx.conf

roles/nginx/files/nginx.conf

@@ -57,7 +57,7 @@ http {
     # Avoids the issue where a non-gzip capable client (which is extremely rare
     # today) would display gibberish if their proxy gave them the gzipped version.
     gzip_vary on;
-    gzip_comp_level 6;
+    gzip_comp_level 5;
     gzip_min_length 860;
     gzip_disable "msie6";
     gzip_http_version 1.1;