Simplified the Travis configuration

SwarrotBundle does not perform rabbitmq calls in its testsuite, so it is not necessary to use different librabbitmq versions.
this makes the Travis build faster by using only 4 jobs rather than 8 (a github orga can only run 5 concurrent jobs on Travis currently)

thus, I'm not even sure it was actually using different versions of librabbitmq as the conf uses the precompiled amqp extension available on Travis, which probably links against a librabbitmq installed elsewhere