Fix interoperability issues with draft-11

[dansarie]

Fix a couple of mentions of draft8 that should be draft11, plus an additional typo in the README. The issues were found during the IETF 121 Hackathon in Dublin.

[dansarie]

The pull request now also fixes the interoperability issues with draft-11 clients. They were due to two different issues:

• First, the SRV and VER tags were in the wrong order in src/tag.rs, causing errors when they were encountered in the correct order in requests. A more robust fix would probably be to compare tags by uint32 value when ordering them, since this will work even for unknown tags. The root cause of this error seems to be that the tags are ordered incorrectly in the draft (see Correct order of tags in table ietf-wg-ntp/draft-roughtime#16).
• Responses did not include NONC tags, as required by the draft.

[int08h]

Thank you so much for the draft11 fixes!