Expanded which versions of zeromq we test against

There seem to be quite a few people now and then that suffer from failing builds. To improve the testability a bit, I've expanded the environments we test against to cover every released zeromq v3 version out there. All versions pass.

Unfortunately, at the present time, travis-ci only lets one test against "32-bit Ubuntu Linux 12.04 (server edition)" (source: http://about.travis-ci.org/docs/user/ci-environment/#CI-environment-OS ). We can only hope that some day other operating systems will be added.