Merge pull request #250 from pabloh/check_for_conf_defined

Check that Rails.application is defined before calling

lib/premailer/rails/css_loaders/asset_pipeline_loader.rb

@@ -25,6 +25,7 @@ def file_name(url)
 
         def asset_pipeline_present?
           defined?(::Rails) &&
+            ::Rails.respond_to?(:application) &&
             ::Rails.application &&
             ::Rails.application.respond_to?(:assets_manifest) &&
             ::Rails.application.assets_manifest