[stable-v1] Fix missing rack/session file

[pboling]

The stable-v1 README's "Usage" section states:

In your config.ru:

# config.ru

require 'rack/session' # <====== 💣💣💣 LoadError! 💣💣💣 
use Rack::Session::Cookie,
   :domain => 'mywebsite.com',
   :path => '/',
   :expire_after => 3600*24,
   :secret => '**unique secret key**'

Following the readme will break any app you try it in!

This PR fixes v1 so it works the same way as v2.

The alternative to this would be to add a rack/session file in a new legacy release of Rack v2 (it is autoloaded, so it actually doesn't have one, which is why this is an issue in the first place).

[ioquatix]

I'm okay with this.