Minitest should be a development dependency

github-markup.gemspec

@@ -17,5 +17,5 @@ Gem::Specification.new do |s|
   s.require_paths = %w[lib]
 
   s.add_dependency "posix-spawn", "~> 0.3.8"
-  s.add_dependency 'minitest', '~> 5.4.3'
+  s.add_development_dependency 'minitest', '~> 5.4.3'
 end