Added missing dependency

Author: fbacall

tess_api_client.gemspec

@@ -14,5 +14,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake'
   s.add_development_dependency 'webmock'
   s.add_development_dependency 'vcr', '~> 3.0.3'
+  s.add_development_dependency 'test-unit', '~> 3.5.3'
   s.add_development_dependency 'simplecov'
 end