Bump nokogiri from 1.6.1 to 1.6.8.1

[dependabot]

Bumps nokogiri from 1.6.1 to 1.6.8.1.

Changelog

Sourced from nokogiri's changelog.

1.6.8.1 / 2016-10-03

Dependency License Notes

Removes required dependency on the pkg-config gem. This dependency
was introduced in v1.6.8 and, because it's distributed under LGPL, was
objectionable to many Nokogiri users (#1488, #1496).

This version makes pkg-config an optional dependency. If it's
installed, it's used; but otherwise Nokogiri will attempt to work
around its absence.

1.6.8 / 2016-06-06

Security Notes

[MRI] Bundled libxml2 is upgraded to 2.9.4, which fixes many security issues. Many of these had previously been patched in the vendored libxml 2.9.2 in the 1.6.7.x branch, but some are newer.

See these libxml2 email posts for more:

• https://mail.gnome.org/archives/xml/2015-November/msg00012.html
• https://mail.gnome.org/archives/xml/2016-May/msg00023.html

For a more detailed analysis, you may care to read Canonical's take on these security issues:

• http://www.ubuntu.com/usn/usn-2994-1

[MRI] Bundled libxslt is upgraded to 1.1.29, which fixes a security issue as well as many long-known outstanding bugs, some features, some portability improvements, and general cleanup.

See this libxslt email post for more:

• https://mail.gnome.org/archives/xslt/2016-May/msg00004.html

Features

Several changes were made to improve performance:

• [MRI] Simplify NodeSet#to_a with a minor speed-up. (#1397)
• XML::Node#ancestors optimization. (#1297) (Thanks, Bruno Sutic!)
• Use Symbol#to_proc where we weren't previously. (#1296) (Thanks, Bruno Sutic!)
• XML::DTD#each uses implicit block calls. (Thanks, @​glaucocustodio!)
• Fall back to the pkg-config gem if we're having trouble finding the system libxml2. This should help many FreeBSD users. (#1417)
• Set document encoding appropriately even on blank document. (#1043) (Thanks, @​batter!)

Bug Fixes

... (truncated)

Commits

• 002e4d8 version bump to v1.6.8.1
• 3b9ee4b update CHANGELOG for pkg-config
• f2dd079 Make the pkg-config gem optional.
• 6b05c5a update v1.6.8 date in CHANGELOG
• 1103a64 version bump to v1.6.8 final
• 03d4022 update CHANGELOG with libxml2 USN info
• a7da0f9 make sure test_all will errexit
• 8d4ea86 make sure test_all doesn't require libxml-ruby
• 80e800b use eval_gemfile to load libxml-ruby
• 95ba49a test_all uses Bundler 1.12.*
• Additional commits viewable in compare view

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