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/tickets/_ticket.html.haml

@@ -5,7 +5,7 @@
         %h4.media-heading
           = ticket.title
         %h5.media-heading
-        -if !ticket.description.blank?
+        - unless ticket.description.blank?
           = markdown(ticket.description)
   %td.col-sm-1.col-md-1
     - if ticket.bought?(current_user)