backport not dumping schema after migrations in prod from Rails 4.1

slightly different implementation, because it's easier to
monkeypatch it like this, rather than redefine entire methods

test plan:
 * remove db/structure.sql
 * run rake db:migrate in development mode; db/structure.sql should
   regenerate
 * remove it again
 * add `config.active_record.dump_schema_after_migration = false` to
   config/environments/development-local.rb
 * run rake db:migrate in development mode again; db/structure.sql
   should not regenerate

Change-Id: I13e85070b9adeac48e5f31a1548a683081cf934e
Reviewed-on: https://gerrit.instructure.com/41891
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>