Decrease Travis CI build duration

* Try and build against the pre-built version of ØMQ, which are stored
  in a separate support repo[0]
* If there isn't a matching pre-build version of ØMQ, then download and
  build it as before
* Fork the run-tests.sh script to travis/script.sh to make it clear that
  the Travis CI build is distinct

[0] https://github.com/phuedx/php-zmq-travis-support