Update minimum version of nokogiri

Fixes: ActionView::Template::Error (undefined method `add_class' for #<Nokogiri::XML::Element:0x0055dbff5252e8>

Gemfile

@@ -130,7 +130,7 @@ gem 'asciidoctor-plantuml', '0.0.8'
 gem 'rouge', '~> 3.1'
 gem 'truncato', '~> 0.7.9'
 gem 'bootstrap_form', '~> 2.7.0'
-gem 'nokogiri', '~> 1.8.2'
+gem 'nokogiri', '~> 1.8.4'
 gem 'escape_utils', '~> 1.1'
 
 # Calendar rendering

Gemfile.lock

@@ -1059,7 +1059,7 @@ DEPENDENCIES
   nakayoshi_fork (~> 0.0.4)
   net-ldap
   net-ssh (~> 5.0)
-  nokogiri (~> 1.8.2)
+  nokogiri (~> 1.8.4)
   oauth2 (~> 1.4)
   octokit (~> 4.9)
   omniauth (~> 1.8)