roles/nginx: Decrease gzip_min_length from 860 to 256

There was some knowledge floating around that 860 bytes was the
optimal size, I think it was from an Akamai engineer or something,
but the HTML 5 Boilerplate server configs use 256 bytes, and I
actually have HTML content that is less than 860 bytes, so I guess
I could benefit from compressing it. gzip compression is costly
for the compression side, but very quick for the client, so this
is a good thing.

See: https://github.com/h5bp/server-configs-nginx/blob/master/nginx.conf

roles/nginx/files/nginx.conf

@@ -63,7 +63,11 @@ http {
     # 75% reduction for most ASCII files (almost identical to level 9).
     gzip_comp_level 5;
 
-    gzip_min_length 860;
+    # Don't compress anything that's already small and unlikely to shrink much
+    # if at all (the default is 20 bytes, which is bad as that usually leads to
+    # larger files after gzipping).
+    gzip_min_length 256;
+
     gzip_disable "msie6";
     gzip_http_version 1.1;
     # Compress all output labeled with one of the following MIME-types.