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] <[email protected]>

Author: dependabot-preview[bot]

sensu-plugins-elasticsearch.gemspec

@@ -35,7 +35,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   s.add_runtime_dependency 'aws-es-transport', '~> 0.1'
   s.add_runtime_dependency 'aws-sdk', ['>= 2.1.14', '< 2.5', '~> 2.1']
   s.add_runtime_dependency 'elasticsearch',     '~> 1.0.14'
-  s.add_runtime_dependency 'rest-client', '1.8.0'
+  s.add_runtime_dependency 'rest-client', '2.1.0'
   s.add_runtime_dependency 'sensu-plugin', '~> 4.0'
 
   s.add_development_dependency 'bundler',                   '~> 1.7'