Log Message:
nokogiri: update to 1.8.1.

This version is necessary for ruby-mini_portile2 2.3.0 in pkgsrc-2017Q3.

pkgsrc changes:
- strict dependency against ruby-mini_portile2 as defined in the Gemfile
- take maintainership

Upstream changes (from CHANGELOG.md):

# 1.8.1 / 2017-09-19

## Dependencies

* [MRI] libxml2 is updated from 2.9.4 to 2.9.5.
* [MRI] libxslt is updated from 1.1.29 to 1.1.30.
* [MRI] optional dependency on the pkg-config gem has had its constraint \ 
loosened to `~> 1.1` (from `~> 1.1.7`). [#1660]
* [MRI] Upgrade mini_portile2 dependency from `~> 2.2.0` to `~> 2.3.0`, \ 
which will validate checksums on the vendored libxml2 and libxslt tarballs \ 
before using them.

## Bugs

* NodeSet#first with an integer argument longer than the length of the NodeSet \ 
now correctly clamps the length of the returned NodeSet to the original length. \ 
[#1650] (Thanks, @Derenge!)
* [MRI] Ensure CData.new raises TypeError if the `content` argument is not \ 
implicitly convertible into a string. [#1669]