Add `test-unit` as development dependency

Fixes `require': cannot load such file -- test/unit (LoadError)

application_insights.gemspec

@@ -25,4 +25,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'yard', '~> 0.8.7.6'
   spec.add_development_dependency 'redcarpet', '~> 3.2.2'
   spec.add_development_dependency 'rack', '>= 1.0.0'
+  spec.add_development_dependency 'test-unit', '~> 3.0.8'
 end