chore: add rackup to sinatra example Gemfile #2449

Job	Run time
mysql / mysql2 / ubuntu-latest	1m 15s
mysql / trilogy / ubuntu-latest	1m 9s
kafka / racecar / ubuntu-latest	15m 2s
kafka / rdkafka / ubuntu-latest	15m 14s
postgresql / pg / ubuntu-latest	2m 58s
kafka / ruby_kafka / ubuntu-latest	5m 37s
postgresql / que / ubuntu-latest	1m 56s
rabbitmq / bunny / ubuntu-latest	59s
other / dalli / ubuntu-latest	2m 31s
redis / redis / ubuntu-latest	2m 5s
other / mongo / ubuntu-latest	2m 13s
redis / resque / ubuntu-latest	1m 51s
redis / sidekiq / ubuntu-latest	6m 12s
	59m 2s

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