nghttp2: bump to 1.31.0

nghttp2: bump to 1.31.0

6e744662 Update bash_completion
478eac09 Update manual pages
88e2029e Bump up version number to 1.31.0, LT revision to 30:0:16
45d76cf5 nghttpx: Close listening socket on graceful shutdown
54573f28 Merge pull request #1137 from nghttp2/session-set-user-data
17793e99 Add nghttp2_session_set_user_data() public API function
5eac3c90 Update manual pages
e70195ae nghttpx: Update doc
fe51e7fa Merge pull request #1130 from nghttp2/avoid-inet_pton-macro
eb951c2c src: Define nghttp2_inet_pton wrapper to avoid inet_pton macro
39f0ce7c Merge pull request #1126 from nghttp2/nghttpx-expired-client-cert
65157811 Merge pull request #1123 from nghttp2/mruby-client-cert-not-before-after
e8af7afc nghttpx: Add an option to accept expired client certificate
38abfd18 nghttpx: Add mruby tls_client_not_before, and tls_client_not_after
ff3edc09 nghttpx: Fix potential memory leak
0bb15406 Bump up version number to 1.31.0-DEV

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>