added missing curb dep

Author: mojodna

Rakefile

@@ -19,6 +19,7 @@ begin
     s.add_development_dependency(%q<mocha>, [">= 0.9.8"])
     s.add_development_dependency(%q<typhoeus>, [">= 0.1.13"])
     s.add_development_dependency(%q<em-http-request>)
+    s.add_development_dependency(%q<curb>, [">= 0.6.6.0"])
   end
   Jeweler::GemcutterTasks.new
 rescue LoadError