zmq.hpp claims to be 4.7.0 when it isn't

[Yichao Yu]

The zmq.hpp appears to be using a pre-released version of `zmq.hpp` that has the version set to `4.7.0` while not containing all the features in that version. In particular, this does not include https://github.com/zeromq/cppzmq/commit/a3e5b54c3cad9dc1c4e86555e47365c2d3950e63. Since the commit introduces a deprecation warning that recommends a new feature introduced in the same commit, attempting to fix it using a version check causes a compilation error on Ubuntu 20.04 due to missing feature.