updating feature docs to reflect capybara 2.3.0 dependency

change made in 8254ddc10c8e34912300ca2c8df275cc5132d1ba
One idea: we could expend the version checker to accept a `minimum_version` and a `recommended_version`.  If the version being used is `<` the min version we'd give the user an error.  If it's `>=` the min version but `<` the recommended version we'd give them a warning recommending they upgrade.

This would communicate the "upgrade to 2.3.0" message we are trying to get across without being a breaking change, and we can continue to update the recommended version as often as we want.  The min version will have to stay at 2.2.0 for all RSpec 3.x releases, though.