Use RSpec::Core::Version instead of RSpec::Version

because capybara/rspec requires rspec-core, but not rspec.

lib/capybara/rspec/features.rb

@@ -1,4 +1,4 @@
-if RSpec::Version::STRING.to_f >= 3.0
+if RSpec::Core::Version::STRING.to_f >= 3.0
   RSpec.shared_context "Capybara Features", :capybara_feature => true do
     instance_eval do
       alias background before
@@ -42,4 +42,4 @@ else
   end
 
   RSpec.configuration.include Capybara::Features, :capybara_feature => true
-end
+end