update dependencies

* use the stock upstream roxml
* upstream roxml says it supports active support 2.x but it doesn't, so
  manually force 3.0.x
* specify a version of nokogiri that we know works

Rakefile

@@ -64,8 +64,10 @@ spec = Gem::Specification.new do |s|
     "tasks/**/*.rb",
     "tasks/**/*.rake"
   ]
-  s.add_dependency('yob-roxml', '>=3.1.6')
+  s.add_dependency('roxml', '~>3.1.6')
+  s.add_dependency('activesupport', '~> 3.0.5')
   s.add_dependency('andand')
+  s.add_dependency('nokogiri', '>=1.4')
 end
 
 Rake::GemPackageTask.new(spec) do |p|