Update github-markup requirement from ~> 1.3 to ~> 4.0

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](github/markup@v1.3.0...v4.0.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

sensu-plugins-elasticsearch.gemspec

@@ -40,7 +40,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
 
   s.add_development_dependency 'bundler',                   '~> 2.1'
   s.add_development_dependency 'codeclimate-test-reporter', '~> 0.4'
-  s.add_development_dependency 'github-markup',             '~> 1.3'
+  s.add_development_dependency 'github-markup',             '~> 4.0'
   s.add_development_dependency 'kitchen-docker',            '~> 2.6'
   # locked to keep ruby 2.1 support, this is pulled in by test-kitchen
   s.add_development_dependency 'mixlib-shellout',           ['< 2.3.0', '~> 2.2']