rails 4.2: remove not-null constraint on brand config variables

since it defaults to an empty hash, it doesn't bother
serializing one when it isn't needed

refs #CNVS-26056

Change-Id: I2515257c8c58aabd26c50c18330361e49cd8b0b1
Reviewed-on: https://gerrit.instructure.com/71133
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

db/migrate/20160129144155_remove_not_null_on_variables.rb

@@ -0,0 +1,11 @@
+class RemoveNotNullOnVariables < ActiveRecord::Migration
+  tag :predeploy
+
+  def up
+    change_column_null :brand_configs, :variables, true
+  end
+
+  def down
+    change_column_null :brand_configs, :variables, false
+  end
+end