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-aws.gemspec

@@ -40,7 +40,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   # https://github.com/fog/fog-core/issues/206
   s.add_runtime_dependency 'fog-core',          '1.45.0'
   s.add_runtime_dependency 'nokogiri',          ['>= 1.10.4', '< 2.0']
-  s.add_runtime_dependency 'rest-client',       '1.8.0'
+  s.add_runtime_dependency 'rest-client',       '2.1.0'
   s.add_runtime_dependency 'right_aws',         '3.1.0'
 
   s.add_development_dependency 'bundler',                   '~> 2.1'