Update github-markup requirement from ~> 1.3 to ~> 3.0

Updates the requirements on [github-markup](https://github.com/github/markup) to permit the latest version.
- [Release notes](https://github.com/github/markup/releases)
- [Changelog](https://github.com/github/markup/blob/master/HISTORY.md)
- [Commits](https://github.com/github/markup/commits/v3.0.1)

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

Author: dependabot-support

sensu-plugins-postgres.gemspec

@@ -41,7 +41,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
 
   s.add_development_dependency 'bundler',                   '~> 1.7'
   s.add_development_dependency 'codeclimate-test-reporter', '~> 1.0'
-  s.add_development_dependency 'github-markup',             '~> 1.3'
+  s.add_development_dependency 'github-markup',             '~> 3.0'
   s.add_development_dependency 'kitchen-docker',            '~> 2.6'
   s.add_development_dependency 'kitchen-localhost',         '~> 0.3'
   # locked to keep ruby 2.1 support, this is pulled in by test-kitchen