Update github-markup requirement from ~> 3.0 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@v3.0.0...v4.0.0)

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

Author: dependabot-preview[bot]

sensu-plugins-filesystem-checks.gemspec

@@ -38,7 +38,7 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'bundler',                   '~> 2.1'
   s.add_development_dependency 'codeclimate-test-reporter', '~> 0.4'
-  s.add_development_dependency 'github-markup',             '~> 3.0'
+  s.add_development_dependency 'github-markup',             '~> 4.0'
   s.add_development_dependency 'pry',                       '~> 0.10'
   s.add_development_dependency 'rake',                      '~> 13.0'
   s.add_development_dependency 'redcarpet',                 '~> 3.2'