Fix to integrate easier with cucumber_rails

Hi,

Thanks for awesome work. I tested the rake task and with a small modification, it works great.

I included `require 'active_support/core_ext/array/grouping' ` in the task file because for some people Rails is not included at the moment when the task starts executing.

I also updated the README to reflect better what `common/std_opts` variable is. (`common` is name from official cucumber.yml docs.)

Let me know what you think.

Cheers,

Strika