roles/nginx: Set gzip_comp_level to 6

Seems to be the sweet spot, as gzip itself defaults to this.

Signed-off-by: Alan Orth <alan.orth@gmail.com>

roles/nginx/files/nginx.conf

@@ -30,6 +30,7 @@ http {
 
     gzip on;
     gzip_vary on;
+    gzip_comp_level 6;
     gzip_min_length 860;
     gzip_disable "msie6";
     gzip_http_version 1.1;