Update pg requirement from = 1.1 to = 1.2.1

Updates the requirements on [pg](https://github.com/ged/ruby-pg) to permit the latest version.
- [Release notes](https://github.com/ged/ruby-pg/releases)
- [Changelog](https://github.com/ged/ruby-pg/blob/master/History.rdoc)
- [Commits](ged/ruby-pg@v1.1.0...v1.2.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

sensu-plugins-postgres.gemspec

@@ -37,7 +37,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   s.add_runtime_dependency 'sensu-plugin', '~> 1.2'
 
   s.add_runtime_dependency 'dentaku',      '2.0.4'
-  s.add_runtime_dependency 'pg',           '1.1'
+  s.add_runtime_dependency 'pg',           '1.2.1'
 
   s.add_development_dependency 'bundler',                   '~> 1.7'
   s.add_development_dependency 'codeclimate-test-reporter', '~> 1.0'