Integration tests failing for librdkafka < 1.0.0

[didier-wenzek]

See pull request Build & test code on Travis CI #13

• The integration tests are ok for all the versions from 1.0.0.RC2 up to 1.3.0
• There is no 1.0.0.RC1 and the previous version was 0.11.6. The tests are failing on an Assert_failure("test/integration/integration.ml", 181, 3)
• The version installed with ubuntu 0.11.3 leads to a time out in the same place related to producers' delivery callbacks.