update to rubocop 0.40

Author: eheydrick

sensu-plugins-postgres.gemspec

@@ -45,6 +45,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rake',                      '~> 10.0'
   s.add_development_dependency 'redcarpet',                 '~> 3.2'
   s.add_development_dependency 'rspec',                     '~> 3.1'
-  s.add_development_dependency 'rubocop',                   '0.34.2'
+  s.add_development_dependency 'rubocop',                   '~> 0.40.0'
   s.add_development_dependency 'yard',                      '~> 0.8'
 end