style: if ! changed by unless

- if ! changed by unless as it is more readable
- !object.blank? is equivalent to object.present?, also more readable

app/views/conference/_conference_details.html.haml

@@ -13,7 +13,7 @@
           - if conference.venue
             %p
               = "#{conference.venue.city}/#{conference.venue.country_name}"
-          - if !conference.description.blank?
+          - unless conference.description.blank?
             %p
               = markdown(conference.description)
         .col-md-2