Update dentaku requirement from = 3.3.4 to = 3.4.2

Updates the requirements on [dentaku](https://github.com/rubysolo/dentaku) to permit the latest version.
- [Release notes](https://github.com/rubysolo/dentaku/releases)
- [Changelog](https://github.com/rubysolo/dentaku/blob/master/CHANGELOG.md)
- [Commits](rubysolo/dentaku@v3.3.4...v3.4.2)

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

Author: dependabot-preview[bot]

sensu-plugins-postgres.gemspec

@@ -38,7 +38,7 @@ Gem::Specification.new do |s|
 
   s.add_runtime_dependency 'sensu-plugin', '~> 4.0'
 
-  s.add_runtime_dependency 'dentaku',      '3.3.4'
+  s.add_runtime_dependency 'dentaku',      '3.4.2'
   s.add_runtime_dependency 'pg',           '1.2.3'
 
   s.add_development_dependency 'bundler',                   '~> 2.1'