Default requires from rails_helper are not in a well-defined order.

If spec/support/helpers.rb gets loaded before spec/support/helpers/session_helpers.rb, then rspec will fail because ``module Features`` will not yet be defined.  On MacOS X the order in which directory entries is read is well defined, but not so on Linux so there is no guarantee on Linux that the support files will be loaded in the correct order.  This one line change is sufficient to correct the problem but may not be the best solution -- one alternative would be to load spec/support/helpers/*.rb from spec/support/helpers.rb, for instance.