Require ActiveSupport from test_helper so key-value and memoize tests are run again

Latest test runs are skipping memoize/cache and key-value tests due to this, because they are only included if I18n::TestCase.key_value? returns true, and it is returning false due to it no being required.

The tests run are now `1272 runs, 1755 assertions` where before they were `1077 runs, 1524 assertions`.

Also add this missing dependency on default Gemfile.