pin rubocop in gemspec

Author: Matt Jones

sensu-plugins-postgres.gemspec

@@ -41,7 +41,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'pg',           '0.18.2'
 
   s.add_development_dependency 'codeclimate-test-reporter', '~> 0.4'
-  s.add_development_dependency 'rubocop',                   '~> 0.30'
+  s.add_development_dependency 'rubocop',                   '0.30'
   s.add_development_dependency 'rspec',                     '~> 3.1'
   s.add_development_dependency 'bundler',                   '~> 1.7'
   s.add_development_dependency 'rake',                      '~> 10.0'