Added ruby2_keywords for Ruby 2.6 and 2.5

lib/delegate/delegate.gemspec

@@ -24,4 +24,6 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+
+  spec.add_runtime_dependency 'ruby2_keywords'
 end