Loosen Nokogiri dependency

Rails 4.2 needs Nokogiri 1.6.0 and above. Loosening this dependency lets us
upgrade apps using Slimmer to Rails 4.2.

Tested successfully with Nokogiri 1.6.0 and 1.6.4.1
Merge pull request #108 from alphagov/loosen_nokogiri_dependency

Loosen Nokogiri dependency