Bundle irb on Ruby 2.6 or later

In Ruby 2.6, irb is shipped as a separeted gem.

Author: cosmo0920

Gemfile

@@ -6,3 +6,4 @@ gemspec
 gem 'simplecov', require: false
 gem 'coveralls', require: false
 gem 'strptime', require: false if RUBY_ENGINE == "ruby" && RUBY_VERSION =~ /^2/
+gem "irb" if RUBY_ENGINE == "ruby" && RUBY_VERSION >= "2.6"