Update README.md

Bump rails to 3.2.10
Fixing tiny typo

Fixes #2377
use respond_{to/with} in promo checkout controller

Fixes #2381
Improves query to fetch products from line_items

This creates 1/2 less queries when fetching producs from an order's line
items
Makes promotion#products always return an array

In case where rules that doesn't respond to #products are associated
with the promotion this method could break with:

  undefined method `flatten' for nil:NilClass

Fixes #2378
use on_hand method to get on_hand for variant while populating order so that :track_inventory_levels is taken into consideration. [Fixes #2379]

Fixes #2382