Fix running dspec task with argument

Rakefile.rb

@@ -35,7 +35,7 @@ task :dspec, [:example_string] => :build_dist do |task, args|
   FileUtils.mkdir_p("test")
   FileUtils.cp("dist/rscons", "test/rscons.rb")
   ENV["dist_specs"] = "1"
-  Rake::Task["spec"].execute(args.example_string)
+  Rake::Task["spec"].execute(args)
   ENV.delete("dist_specs")
   FileUtils.rm_f(Dir.glob(".rscons-*"))
 end