update nokogiri to at least 1.7.1
update nokogiri to at least 1.7.1

Gemnasium raised a [security vulnerability](https://gemnasium.com/github.com/ministryofjustice/peoplefinder/alerts#alert-5052745) related to nokogiri
that can be fixed by version 1.7.1. Other dependencies needed
updating to (rails-dom-testing, html2haml) to allow the nokogiri
update - Conservatively updated.

[Primary fix source](https://github.com/sparklemotion/nokogiri/issues/1615)

ministryofjustice/peoplefinder,ministryofjustice/peoplefinder,ministryofjustice/peoplefinder,ministryofjustice/peoplefinder,ministryofjustice/peoplefinder