Bump Rails to ~> 3.2.18
Bump activemerchant to 1.43.1

This should fix spree/spree_gateway#149
Lock capybara-accessible down to 0.2.0

0.2.1 causes this bug: https://github.com/spree/spree/pull/3414#issuecomment-47186627

Related to #3414
Optimize available_countries

Use .sort_by so that we only run .parameterize once per country.

Fixes #4899