Update minumum nokogiri version.

Author: gkellogg

Gemfile

@@ -9,7 +9,7 @@ group 'development' do
   gem 'rdf-isomorphic', github: "ruby-rdf/rdf-isomorphic",  branch: "develop"
   gem 'rdf-vocab',      github: "ruby-rdf/rdf-vocab",       branch: "develop"
   gem 'sxp',            github: "dryruby/sxp.rb",           branch: "develop"
-  gem 'nokogiri',       '~> 1.10'
+  gem 'nokogiri',       '~> 1.13', '>= 1.13.4'
 end
 
 group :debug do