[ruby/csv] Use test-unit gem instead of test-framework of ruby repo

https://github.com/ruby/csv/commit/9c4add0d31

lib/csv/csv.gemspec

@@ -60,4 +60,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "benchmark_driver"
+  spec.add_development_dependency "test-unit", ">= 3.4.3"
 end