🎉 Support ActiveAdmin 2.x and code style improvements

- Add support ActiveAdmin for 2.x
- Add rubocop config
- Add frozen_string_literal to ruby files
- Apply changes to respect rubocop alerts

lib/formtastic/inputs/quill_editor_input.rb

@@ -1,15 +1,17 @@
+# frozen_string_literal: true
+
 module Formtastic
   module Inputs
     class QuillEditorInput < Formtastic::Inputs::TextInput
       def to_html
         input_wrapping do
           label_html <<
-          template.content_tag( :div, input_html_options.merge( class: 'quill-editor' ) ) do
-            builder.hidden_field( input_name ) <<
-            template.content_tag( :div, class: 'quill-editor-content' ) do
-              object.send( method ).try :html_safe
+            template.content_tag(:div, input_html_options.merge(class: 'quill-editor')) do
+              builder.hidden_field(input_name) <<
+                template.content_tag(:div, class: 'quill-editor-content') do
+                  object.send(method).try :html_safe
+                end
             end
-          end
         end
       end
     end