📌 Add specific version to add_development_dependency

Author: kozo2

rubydown.gemspec

@@ -43,5 +43,5 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler", "~> 1.17"
   spec.add_development_dependency "rake", "~> 10.0"
-  spec.add_development_dependency "test-unit"
+  spec.add_development_dependency "test-unit", "~> 3.3"
 end