Require Autoload module in fast_spec_helper

This prevents the following error when running the tests locally on my computer. Why this error hasn't occurred on Travis CI is yet unknown.
    
```
.../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'
```

Full backtrace available at https://gist.github.com/Koronen/723eabff8d3c68338a21.