"Manually install" the zmq extension.

Currently, very nearly all tests are skipped during the Travis CI
(see https://travis-ci.org/phuedx/php-zmq/jobs/11650067 and
https://travis-ci.org/phuedx/php-zmq/jobs/11650068). The majority of
tests are skipped because the zmq extension hasn't been loaded. Since
manually installing the runkit extension during the Travis CI build for
phuedx/pinkerton works, try manually installing the zmq extension here.