Merge pull request #164 from airbnb/zs--change-required-ruby-version

bump required ruby version

Author: zachsabin

rubocop-airbnb/rubocop-airbnb.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
   spec.license = 'MIT'
   spec.version = RuboCop::Airbnb::VERSION
   spec.platform = Gem::Platform::RUBY
-  spec.required_ruby_version = '>= 2.1'
+  spec.required_ruby_version = '>= 2.3'
 
   spec.require_paths = ['lib']
   spec.files = Dir[