Fix load order issue

Only define `:feature` alias if it's not available. It is possible that Capybara is required before the `FeatureExampleGroup`. It is also possible another gem or a custom `feature` implementation is already available. This does not define our custom alias if `RSpec.feature` already exists.
Do you still want to alias `feature` here if `RSpec.respond_to?(:feature)`?  That could override a user alias, right?  Seems like we should only alias it if it's not already defined., regardless of capybara version.