Exclude conference model from ClassLength cop

Exclude conference.rb file from ClassLength Rubocop cop and decrease the
max value of this cop to 300, as the following class in number of lines is
event.rb with 218.

Also, remove unused comments from Conference model class.

.rubocop.yml

@@ -95,7 +95,9 @@ Metrics/BlockNesting:
   Max: 4
 
 Metrics/ClassLength:
-  Max: 575
+  Max: 300
+  Exclude: 
+    - 'app/models/conference.rb'
 
 # avoid redundunt curly braces when it is obvious that hash is used
 Style/BracesAroundHashParameters:

app/models/conference.rb

@@ -1,6 +1,3 @@
-# rubocop:disable Metrics/ClassLength
-##
-# This class represents a conference
 class Conference < ActiveRecord::Base
   require 'uri'
   serialize :events_per_week, Hash
@@ -1081,4 +1078,3 @@ class Conference < ActiveRecord::Base
     result
   end
 end
-# rubocop:enable Metrics/ClassLength