- Version umpdate to 1.31.1:
* Fix bsc#1088639 CVE-2018-1000168
* https://nghttp2.org/blog/2018/04/12/nghttp2-v1-31-1/

- Version update to 1.31.0:
* lib: Add nghttp2_session_set_user_data() public API function (GH-1137)
* src: Define nghttp2_inet_pton wrapper to avoid inet_pton macro (GH-1128)
* nghttpx: Close listening socket on graceful shutdown
* nghttpx: Add an option to accept expired client certificate (GH-1126)
* nghttpx: Add mruby tls_client_not_before, and tls_client_not_after (GH-1123)
* nghttpx: Fix potential memory leak
* lib: Allow PING frame to be sent after GOAWAY (GH-1103)
* nghttpx: Fix bug that h1 backend idle timeout expires sooner
* nghttpx: Stop overwrite of first header on mruby call to env.req.set_header(..) (Patch from Dylan Plecki) (GH-1119)
* nghttpx: Add upgrade-scheme parameter to backend option (GH-1099)
* nghttpx: Fix missing ALPN validation (--npn-list) (GH-1094)
* nghttpx: Remember which resource is pushed for RFC 8297 (GH-1101)

- Drop spdylay dependency as it is deprecated since version 1.28.0
and removed from cofnigure.ac since 1.29.0