Handle situations when Rails module is defined without env

Gems like rails-html-sanitizer open Rails namespace without
requiring Rails. Instead check if Rails.env is defined.

Similar to: rails/activeresource#216

lib/wicked_pdf.rb

@@ -96,7 +96,7 @@ def pdf_from_url(url, options = {})
   private
 
   def in_development_mode?
-    return Rails.env == 'development' if defined?(Rails)
+    return Rails.env == 'development' if defined?(Rails.env)
     RAILS_ENV == 'development' if defined?(RAILS_ENV)
   end