Update rubocop, add appraisal gem

spec/telegram/bot/client_stub_spec.rb

@@ -45,7 +45,7 @@ RSpec.describe Telegram::Bot::ClientStub do
     end
 
     context 'when username and token are given' do
-      let(:args) { %w(token superbot) }
+      let(:args) { %w[token superbot] }
       its(:token) { should eq args[0] }
       its(:username) { should eq args[1] }
     end