Add PG gem dep

Author: tas50

sensu-plugins-postgres.gemspec

@@ -31,6 +31,7 @@ Gem::Specification.new do |s|
   s.required_ruby_version  = '>= 1.9.3'
 
   s.add_runtime_dependency 'sensu-plugin', '1.1.0'
+  s.add_runtime_dependency 'pg', '0.18.2'
 
   s.add_development_dependency 'codeclimate-test-reporter', '~> 0.4'
   s.add_development_dependency 'rubocop',                   '~> 0.30'