add the rubocop gem

Author: svevang

service_monitor.gemspec

@@ -36,5 +36,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "pry", "~> 0.11"
   spec.add_development_dependency "rake", "~> 10.0"
   spec.add_development_dependency "rspec", "~> 3.0"
+  spec.add_development_dependency "rubocop", "~> 0.55.0"
   spec.add_development_dependency "timecop", "~> 0.9"
 end