[pkg] downgrade pyzmq version requirement

It is not clear why the pinning was made in the first place, and we need
to downgrade because the current pyzmq version packaged for jessie is
14.4.0. Pinning to a higher version that is not available in debian
causes problems when using python entrypoints for Soledad Server (as we
do now).

pkg/requirements.pip

 jsonschema
 pyopenssl
 python-dateutil
-pyzmq>=14.4.1
+pyzmq>=14.4.0
 txzmq>=0.7.3
 -e .