Upgrade Nokogiri from 1.6.7.2 to 1.6.8
Upgrade Nokogiri from 1.6.7.2 to 1.6.8

Fix for CVE-2015-8806 (denial of service or RCE from libxml2 and
libxslt).

alphagov/policy-publisher,alphagov/policy-publisher,alphagov/policy-publisher,alphagov/policy-publisher