Fix incompatibility with new minitest and mocha

Author: MassimilianoLattanzio

active_shipping.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |s|
   s.add_dependency("active_utils", "~> 3.3.1")
   s.add_dependency("nokogiri", ">= 1.6")
 
-  s.add_development_dependency("minitest")
+  s.add_development_dependency("minitest", "~> 5.18.0")
   s.add_development_dependency("minitest-reporters")
   s.add_development_dependency("rake")
   s.add_development_dependency("mocha", "~> 1")