Build php-zmq on Travis CI

### Preamble

@hintjens announced ØMQ v4 recently (see https://twitter.com/hintjens/status/381068131264761856). Soon after, I tried to build **php-zmq** against it. It failed and so I resolved to submit a PR with the fix. First though, I'd like to be confident that I, and other contributors, don't break the build.

### Build php-zmq on Travis CI

This PR introduces a Travis build configuration that builds **php-zmq** for the latest versions of PHP 5.4 and 5.5 against the following versions of ØMQ:

* 2.2.0
* 3.1.0
* 3.2.0
* 3.2.1
* 3.2.2
* 3.2.3
* 3.2.4
* 4.0.0

You can see an example of a genuine failed build here: https://travis-ci.org/phuedx/php-zmq/builds/11708039

### Breaking changes

None.

### Notes

* You'll need to enable the Travis CI service hook for the **php-zmq** repository (see https://travis-ci.org/profile)
* I've noticed that **php-zmq** doesn't build against ØMQ versions 2.2.0 through 3.2.0, which, you'll notice, can be disabled easily should you wish
* Unfortunately, it took me a few attempts to get **php-zmq** building on Travis CI, which has resulted in far-from-ideal history