updating feature docs to reflect capybara 2.3.0 dependency

change made in 8254ddc10c8e34912300ca2c8df275cc5132d1ba
This seems like a breaking change -- users who were on RSpec 3.0.0 and capabara 2.2.0 will get errors when they upgrade to the RSpec 3.1 (or whatever release includes this version bump).

If you simply can't use capybara 2.2.0 against RSpec 3 (in which case, this won't be breaking for anyone since they can't be using 2.2.0) this is fine, but if it's just that 2.3.0 is recommended I don't think this is OK.