Upgrade zeromq-src to 0.3

This release includes a fix for cross-compiling, and bumps the ZeroMQ
source to v4.3.5.

Author: MarijnS95

zmq-sys/Cargo.toml

@@ -20,7 +20,7 @@ libc = "0.2.15"
 
 [build-dependencies]
 system-deps = "6"
-zeromq-src = { version = "0.2.1" }
+zeromq-src = "0.3"
 
 [package.metadata.system-deps]
 libzmq = "4.1"