Make implicit minimum Tornado version explicit
Make implicit minimum Tornado version explicit

TornadoTransportTests.test__sending_successfully_calls_success_callback uses gen.maybe_future, which appeared in Tornado 4.0
TornadoAsyncClientTestCase.test_sending_to_unresponsive_sentry_server_logs_error and TornadoTransportTests.test__sending_with_error_calls_error_callback use gen.sleep, which appeared in Tornado 4.1
getsentry/raven-python,johansteffner/raven-python,getsentry/raven-python,johansteffner/raven-python,johansteffner/raven-python,getsentry/raven-python