Fix circular require

"capybara/dsl.rb" already requires "capybara.rb". We also update
"capybara/cucumber.rb" to catch up with this change.

lib/capybara.rb

@@ -337,7 +337,6 @@ module Capybara
 
   require 'capybara/helpers'
   require 'capybara/session'
-  require 'capybara/dsl'
   require 'capybara/window'
   require 'capybara/server'
   require 'capybara/selector'

lib/capybara/cucumber.rb

@@ -1,4 +1,4 @@
-require 'capybara'
+require 'capybara/dsl'
 require 'capybara/rspec/matchers'
 
 World(Capybara::DSL)