Fix `#down` for two protected branches-related migrations.

- The migrations called `add_column_with_default` with a `null` option,
  which the Rails `add_column` method accepts. This fails because
  `add_column_with_default` expects an `allow_null` option instead.

- The migrations have been fixed to use `allow_null`.

db/migrate/20160705055809_remove_developers_can_push_from_protected_branches.rb

@@ -14,6 +14,6 @@ class RemoveDevelopersCanPushFromProtectedBranches < ActiveRecord::Migration
   end
 
   def down
-    add_column_with_default(:protected_branches, :developers_can_push, :boolean, default: false, null: false)
+    add_column_with_default(:protected_branches, :developers_can_push, :boolean, default: false, allow_null: false)
   end
 end

db/migrate/20160705055813_remove_developers_can_merge_from_protected_branches.rb

@@ -14,6 +14,6 @@ class RemoveDevelopersCanMergeFromProtectedBranches < ActiveRecord::Migration
   end
 
   def down
-    add_column_with_default(:protected_branches, :developers_can_merge, :boolean, default: false, null: false)
+    add_column_with_default(:protected_branches, :developers_can_merge, :boolean, default: false, allow_null: false)
   end
 end