drop support for Ruby 2.0; add Ruby 2.3

sensu-plugins-mailgun.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |s|
   s.platform               = Gem::Platform::RUBY
   s.post_install_message   = 'You can use the embedded Ruby by setting EMBEDDED_RUBY=true in /etc/default/sensu'
   s.require_paths          = ['lib']
-  s.required_ruby_version  = '>= 1.9.3'
+  s.required_ruby_version  = '>= 2.1.0'
   s.summary                = 'Sensu plugins for mailer'
   s.test_files             = s.files.grep(%r{^(test|spec|features)/})
   s.version                = SensuPluginsMailgun::Version::VER_STRING