Update sensu-plugin requirement from ~> 2.0 to >= 2, < 4

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](https://github.com/sensu-plugins/sensu-plugin/commits/3.0.0)

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

Author: dependabot-support

sensu-plugins-graphite.gemspec

@@ -36,7 +36,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'array_stats',      '0.6.0'
   s.add_runtime_dependency 'ipaddress',        '= 0.8.3'
   s.add_runtime_dependency 'rest-client',      '1.8.0'
-  s.add_runtime_dependency 'sensu-plugin',     '~> 2.0'
+  s.add_runtime_dependency 'sensu-plugin',     '>= 2', '< 4'
   s.add_runtime_dependency 'simple-graphite',  '2.1.0'
 
   s.add_development_dependency 'bundler',                   '~> 1.7'