Add explicit dep for test-unit to gemspec

Otherwise some combinations of rake and bundler fail.

Author: robinst

taglib-ruby.gemspec

@@ -28,6 +28,7 @@ DESC
   s.add_development_dependency 'shoulda-context', '~> 1.0'
   s.add_development_dependency 'yard', '~> 0.7'
   s.add_development_dependency 'kramdown', '~> 1.0'
+  s.add_development_dependency 'test-unit', '~> 3.1'
 
   s.extensions = [
     "ext/taglib_base/extconf.rb",