Resolve bad platform in gemspec

Author: tas50

sensu-plugins-postgres.gemspec

@@ -27,7 +27,7 @@ Gem::Specification.new do |s|
   s.require_paths          = ['lib']
   s.cert_chain             = ['certs/sensu-plugins.pem']
   s.signing_key            = File.expand_path(pvt_key) if $PROGRAM_NAME =~ /gem\z/
-  s.platform               = ruby
+  s.platform               = Gem::Platform::RUBY
   s.required_ruby_version  = '>= 1.9.3'
 
   s.add_development_dependency 'codeclimate-test-reporter'