HTTP2: Update to draft-15

* Add new error code (HTTP_1_1_REQUIRED)
* Use hexa value for error code (like the spec)
* Add reference to Alt-SVC spec

Change-Id: Ifbcb32e97168440cb34f59740871b6c896e90461
Reviewed-on: https://code.wireshark.org/review/4992
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Michael Mann <mmann78@netscape.net>