Increased size of value_* col.

db/migrate/20151115000003_update_history.rb

@@ -0,0 +1,12 @@
+class UpdateHistory < ActiveRecord::Migration
+  def up
+    remove_index :histories, [:value_from]
+    remove_index :histories, [:value_to]
+    change_table(:histories) do |t|
+      t.change :value_from, :text, limit: 500, null: true
+      t.change :value_to, :text, limit: 500, null: true
+    end
+    add_index :histories, [:value_from], length: 255
+    add_index :histories, [:value_to], length: 255
+  end
+end