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
Merge pull request #132 from phuedx/travisBuildPerf

Decrease Travis CI build duration
version error
Merge pull request #136 from wangxx2026/master

version error
Add PHP 5.6
Merge branch 'master' of github.com:mkoppanen/php-zmq