Use InstanceTag / TextArea based on Rails version

Use InstanceTag for Rails 3 and TextArea otherwise. This allows
sir-trevor-rails to work on Rails 4.

This is based on the fix for the same issue contained within
#16

lib/sir-trevor/helpers/form_helper.rb

@@ -14,11 +14,17 @@ def sir_trevor_text_area(object_name, method, options = {})
         input_html[:class] = "sir-trevor-area visuallyhidden"
         hash = input_html.stringify_keys
 
-        instance_tag = ActionView::Base::InstanceTag.new(object_name, method, self, options.delete(:object))
-        instance_tag.send(:add_default_name_and_id, hash)
-
         output_buffer = ActiveSupport::SafeBuffer.new
-        output_buffer << instance_tag.to_text_area_tag(input_html)
+        # support both Rails 3 & 4
+	if defined? ActionView::Base::InstanceTag
+          instance_tag = ActionView::Base::InstanceTag.new(object_name, method, self, options.delete(:object))
+          instance_tag.send(:add_default_name_and_id, hash)
+          output_buffer << instance_tag.to_text_area_tag(input_html)
+	else
+	  instance_tag = ActionView::Helpers::Tags::TextArea.new(object_name, method, self, input_html)
+	  instance_tag.send(:add_default_name_and_id, hash)
+	  output_buffer << instance_tag.render
+	end
 
         output_buffer