Explicitly set null:true for migrations with timestamps

db/migrate/20141130153941_create_problems.rb

@@ -4,7 +4,7 @@ class CreateProblems < ActiveRecord::Migration
       t.string :title
       t.text :content
 
-      t.timestamps
+      t.timestamps null: true
     end
   end
 end

db/migrate/20141214214958_create_problems_again.rb

@@ -1,7 +1,7 @@
 class CreateProblemsAgain < ActiveRecord::Migration
   def change
     create_table :problems do |t|
-      t.timestamps
+      t.timestamps null: true
     end
   end
 end