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.
Allow users to disable database_cleaner hooks

The hooks that automatically invoke database_cleaner before every
scenario can sometimes cause problems when switching cleaning strategies
between scenarios (issue #215).

To allow a user to take full control of when database_cleaner is
invoked, the configuration option
Cucumber::Rails::Database.autorun_database_cleaner is provided,
defaulting to true.