Require `active_support` in fast_spec_helper

This prevents the following error when running the tests.

```
.../activesupport-4.1.5/lib/active_support/number_helper.rb:3:in `<module:NumberHelper>': uninitialized constant ActiveSupport::Autoload (NameError)
   ...
   from .../activesupport-4.1.5/lib/active_support/core_ext.rb:1:in `<top (required)>'
   from .../spec/fast_spec_helper.rb:6:in `require'
```

See also: <https://github.com/rails/rails/issues/14664>.