Need to update github.com/paultag/sniff dependency

[ingvarm-gr]

Due to paultag/sniff#12 and paultag/sniff#9 being merged yester day (2020-02-06, for those reading in the future), one or both of "re-vendor the library" and/or "update go.mod" should be done.

This is tangentially related to #775 (which was fixed, I believe, in the vendored copy of the library in PR #777, but possibly un-done in PR #4012).

I am willing to try to cut a PR for this (should only be two files, after all), but it might be quicker if someone more familiar with the ingress-nginx code base does it.