Remove environment dependency for db:schema:load

All of the behavior :environment was giving (that db:schema:load needed) was
provided as well with :load_config.

This will address an issue introduced in
https://github.com/rails/rails/pull/15394. The fact that db:schema:load now
drops and creates the database causes the Octopus gem to have [an
issue](https://github.com/tchandy/octopus/issues/273) during the drop step for
the test database (which wasn't happening in db:schema:load before). The error
looks like:

    ActiveRecord::StatementInvalid: PG::ObjectInUse: ERROR:  cannot drop the
currently open database
   : DROP DATABASE IF EXISTS "app_test"

Because of the timing, this issue is present in master, 4-2-*, and 4.1.8.

Ping @sgrif