Multiple-gemfile testing and travis configuration

The test suite now runs (and passes!) against the latest versions of:

* Rails 3.0, 3.1 and 3.2
* Capybara 1.1 and 2.0

The default test task is unchanged, and runs only against Rails 3.2. See
the README for details of running all (or selected) tests.

Addresses issues #237 and #238.