nginx-devel: updated to 1.15.8

Changes with nginx 1.15.8:
*) Feature: the $upstream_bytes_sent variable.
*) Feature: new directives in vim syntax highlighting scripts.
*) Bugfix: in the "proxy_cache_background_update" directive.
*) Bugfix: in the "geo" directive when using unix domain listen sockets.
*) Workaround: the "ignoring stale global SSL error ... bad length"
  alerts might appear in logs when using the "ssl_early_data" directive
  with OpenSSL.
*) Bugfix: in nginx/Windows.
*) Bugfix: in the ngx_http_autoindex_module on 32-bit platforms.

(adam)