Add dependency simplecov_json_formatter for development

Author: Umekawa

ruboty-github.gemspec

@@ -28,6 +28,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rubocop-rake'
   spec.add_development_dependency 'rubocop-rspec'
   spec.add_development_dependency 'simplecov'
+  spec.add_development_dependency 'simplecov_json_formatter'
   spec.add_development_dependency 'webmock'
   spec.metadata['rubygems_mfa_required'] = 'true'
 end