Update sensu-plugin requirement from ~> 1.2 to >= 1.2, < 5.0

Updates the requirements on [sensu-plugin](https://github.com/sensu-plugins/sensu-plugin) to permit the latest version.
- [Release notes](https://github.com/sensu-plugins/sensu-plugin/releases)
- [Changelog](https://github.com/sensu-plugins/sensu-plugin/blob/master/CHANGELOG.md)
- [Commits](sensu-plugins/sensu-plugin@v1.2.0...4.0.0)

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

Author: dependabot-preview[bot]

sensu-plugins-postgres.gemspec

@@ -34,7 +34,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   s.test_files             = s.files.grep(%r{^(test|spec|features)/})
   s.version                = SensuPluginsPostgres::Version::VER_STRING
 
-  s.add_runtime_dependency 'sensu-plugin', '~> 1.2'
+  s.add_runtime_dependency 'sensu-plugin', '>= 1.2', '< 5.0'
 
   s.add_runtime_dependency 'dentaku',      '3.3.4'
   s.add_runtime_dependency 'pg',           '1.2.1'