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/models/venue.rb

@@ -44,7 +44,7 @@ class Venue < ActiveRecord::Base
   def generate_guid
     loop do
       @guid = SecureRandom.urlsafe_base64
-      break if !Venue.where(guid: guid).any?
+      break unless Venue.where(guid: guid).any?
     end
     self.guid = @guid
   end