Decrease Travis CI build duration

By building against a pre-built version of ØMQ we can *dramatically* reduce the Travis CI build duration at the cost of increasing the size of the repo. The following is from the email I received about the latest build of [phuedx/php-zmq](https://github.com/phuedx/php-zmq):

> Build Update for phuedx/php-zmq
> -------------------------------------
>
> Build: [#10](https://travis-ci.org/phuedx/php-zmq/builds/28952956)
> Status: Passed
>
> Duration: 9 minutes and 51 seconds

As for the increase in size of the repo: `du -sm /path/to/php-zmq/travis/` reports that the `travis` directory is 79 MB but `git clone https://url/of/php-zmq` reports that it receives 13.19 MB of compressed objects in total.

Alternatively, we could always reduce the size of the Travis CI build matrix…