Log Message:
nokogiri: update to 1.8.3.

Upstream changes (from CHANGELOG.md):

# 1.8.3 / 2018-06-16

## Security Notes

[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

## Dependencies

* [MRI] libxml2 is updated from 2.9.7 to 2.9.8

## Features

* Node#classes, #add_class, #append_class, and #remove_class are added.
* NodeSet#append_class is added.
* NodeSet#remove_attribute is a new alias for NodeSet#remove_attr.
* NodeSet#each now returns an Enumerator when no block is passed
  (Thanks, @park53kr!)
* [JRuby] General improvements in JRuby implementation (Thanks, @kares!)

## Bug fixes

* CSS attribute selectors now gracefully handle queries using integers. [#711]
* Handle ASCII-8BIT encoding on fragment input [#553]
* Handle non-string return values within `Reader` [#898]
* [JRuby] Allow Node#replace to insert Comment and CDATA nodes. [#1666]
* [JRuby] Stability and speed improvements to `Node`, `Sax::PushParser`,
  and the JRuby implementation [#1708, #1710, #1501]