Allow use of capybara-email in Capybara 2

[woahdae]

Capybara 2 does not define 'url' on the selenium driver, which
previously Capybara::Email::Driver#follow relied on. Fortunately,
Capybara::Session#visit is more robust in Capybara 2, and we can rely on
it to attach the correct full path to relative urls instead of
leveraging the previous 'url' method.

[travisbot]

This pull request passes (merged e2aeda0 into 37f21ed).

[bcardarella]

I think the way I'd prefer to handle this is release a 1.0 version of capybara-email and then start a 2.0 version that supports Capybara 2.0. The capybara version dependency in the gemspec can be set to '~> 1 for 1.0 and '~> 2 for 2.0. Thoughts?

[woahdae]

Late to the party, but sounds good!