updating feature docs to reflect capybara 2.3.0 dependency

change made in 8254ddc10c8e34912300ca2c8df275cc5132d1ba
I was on the fence if this should be a breaking change. Given our implicit dependency on capybara I probably should have worked more closely with them before the main release. They appeared to have the gem ready, but didn't push it out until we released.

Technically, 2.2.0 is not RSpec 3 API compliant given the deprecation warnings it spits out. If they had made the change in a 2.2.x patch release we can avoid this. I can try to ping them to see if they are willing to back port it.