Revert libxml2 behavior in Nokogiri gem that could cause XSS

[MRI] Behavior in libxml2 has been reverted which caused
CVE-2018-8048 (loofah gem), CVE-2018-3740 (sanitize gem), and
CVE-2018-3741 (rails-html-sanitizer gem). The commit in question is
here:

https://github.com/GNOME/libxml2/commit/960f0e2

and more information is available about this commit and its impact
here:

https://github.com/flavorjones/loofah/issues/144

This release simply reverts the libxml2 commit in question to protect
users of Nokogiri’s vendored libraries from similar vulnerabilities.

If you’re offended by what happened here, I’d kindly ask that you
comment on the upstream bug report here:

https://bugzilla.gnome.org/show_bug.cgi?id=769760