fix bundler version

Signed-off-by: Todd Campbell <[email protected]>

Author: Todd Campbell

sensu-plugins-elasticsearch.gemspec

@@ -38,7 +38,7 @@ Gem::Specification.new do |s| # rubocop:disable Metrics/BlockLength
   s.add_runtime_dependency 'rest-client', '2.1.0'
   s.add_runtime_dependency 'sensu-plugin', '~> 4.0'
 
-  s.add_development_dependency 'bundler',                   '~> 1.7'
+  s.add_development_dependency 'bundler',                   '~> 2.1'
   s.add_development_dependency 'codeclimate-test-reporter', '~> 0.4'
   s.add_development_dependency 'github-markup',             '~> 1.3'
   s.add_development_dependency 'kitchen-docker',            '~> 2.6'