Update rest-client requirement from = 1.8.0 to = 2.1.0

Updates the requirements on [rest-client](https://github.com/rest-client/rest-client) to permit the latest version.
- [Release notes](https://github.com/rest-client/rest-client/releases)
- [Changelog](https://github.com/rest-client/rest-client/blob/master/history.md)
- [Commits](rest-client/rest-client@v1.8.0...v2.1.0)

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

Author: dependabot-preview[bot]

sensu-plugins-graphite.gemspec

@@ -35,7 +35,7 @@ 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 'rest-client',      '2.1.0'
   s.add_runtime_dependency 'sensu-plugin',     '~> 2.0'
   s.add_runtime_dependency 'simple-graphite',  '2.1.0'