Remove libzmq-dev from dependency list

System installed library of libzmq-dev will have libzmq1,
which doesn't support RCVTIMEO in some linux distros, eg.
ubuntu 12.04 and pip will install libzmq and compile the
supporting version anyway so remove libzmq-dev from dependency
installation by apt-get in README.rst.

Change-Id: Ifd67c7ded5db7dbb82624d2aa08843278f2de72c
Merge "Remove libzmq-dev from dependency list"