Make check conditional to Rails version

This PR depends on action_dispatch.show_detailed_exceptions being set,
which was only introduced in Rails 3.2.1.

Here we introduce a conditional check based on the required Rails
version in order to ensure backwards compatibility.

lib/cucumber/rails/action_controller.rb

@@ -7,7 +7,9 @@ class ActionDispatch::ShowExceptions
 
   def call(env)
     env['action_dispatch.show_exceptions'] = !!ActionController::Base.allow_rescue
-    env['action_dispatch.show_detailed_exceptions'] = !ActionController::Base.allow_rescue
+    if Rails.version >= "3.2.1"
+      env['action_dispatch.show_detailed_exceptions'] = !ActionController::Base.allow_rescue
+    end
     __cucumber_orig_call__(env)
   end
 end