Use only new hash syntax

db/migrate/20121224144728_create_cfp_table.rb

@@ -1,10 +1,10 @@
 class CreateCfpTable < ActiveRecord::Migration
   def up
     create_table :call_for_papers do |t|
-      t.date :start_date, :null => false
-      t.date :end_date, :null => false
-      t.date :hard_deadline, :null => false
-      t.text :description, :null => false
+      t.date :start_date, null: false
+      t.date :end_date, null: false
+      t.date :hard_deadline, null: false
+      t.text :description, null: false
       t.references :conference
 
       t.timestamps