Make sure we are comparing gem versions, closes #1024

Due to rails/rails#8501, we can no longer case the Rails version to a float. This version should hopefully work on both Rails 4 master and on Rails 3.

lib/capybara/rails.rb

@@ -3,7 +3,7 @@
 
 Capybara.app = Rack::Builder.new do
   map "/" do
-    if Rails.version.to_f >= 3.0
+    if Gem::Version.new(Rails.version) >= Gem::Version.new("3.0")
       run Rails.application
     else # Rails 2
       use Rails::Rack::Static