update trex version for constants.so import bug

update trex version for constants.so import bug

Previously before we disabled Trex in the unittests
we were hitting an error when running Python3.5,
see full description in 28209

https://gerrit.opnfv.org/gerrit/#/c/28209/

File "/home/jenkins/opnfv/slave_root/workspace/yardstick-verify-master/trex/scripts/external_libs/pyzmq-14.5.0/python3/fedora18/64bit/zmq/backend/cython/__init__.py", line 6, in <module>
    from . import (constants, error, message, context,
ImportError: cannot import name 'constants'

It looks like this was an actual upstream Trex bug
and it was fixed in

https://github.com/cisco-system-traffic-generator/trex-core/commit/9808fce2b0b3b5b7c9b28303e99486501a1cc10d
zmq: fix wrong constants.so (was used from 3.5 to both versions)
Yaroslav Brustinov <ybrustin@cisco.com>

This Trex fix should be in v2.20.

We don't test Trex in unitests, but people running
Trex on Python3 should hit this issue.

Change-Id: Ic914e48819d7fd3e951628f7c3d3778849da7d94
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>