Make specs pass with recent version of Sinatra

Sinatra is not doing anything wrong, as rack.errors is required
by the Rack SPEC.

spec/sinatra_integration_spec.rb

@@ -49,7 +49,7 @@ def self.get(path, &block)
 describe "Forme Sinatra ERB integration" do
   def sin_get(path)
     s = String.new
-    FormeSinatraTest.new.call(@rack.merge('PATH_INFO'=>path))[2].each{|str| s << str}
+    FormeSinatraTest.new.call(@rack.merge('PATH_INFO'=>path, 'rack.errors'=>$stderr))[2].each{|str| s << str}
     s.gsub(/\s+/, ' ').strip
   end