Improve old version error message. Add note about requried zeromq version.

README.md

@@ -9,6 +9,11 @@ API
 See [API.md](https://github.com/Neopallium/lua-zmq/blob/master/API.md) and
 [ØMQ docs](http://api.zero.mq/2-1-3:_start).
 
+Requirements
+============
+
+* ZeroMQ version 2.1.x.  Might work with some 2.0.x versions (2.0.6 and lower are not supported).
+
 Installation
 ============
 

src/pre_generated-zmq.nobj.c

@@ -1742,7 +1742,7 @@ static const char zmq_ffi_lua_code[] = "-- try loading luajit's ffi\n"
 
 /* detect really old ZeroMQ 2.0.x series. */
 #if !defined(ZMQ_RCVMORE)
-#error "Your version of ZeroMQ is too, old."
+#error "Your version of ZeroMQ is too old.  Please upgrade to version 2.1 or to the latest 2.0.x"
 #endif
 
 typedef void * ZMQ_Socket;

src/socket.nobj.lua

@@ -35,7 +35,7 @@ typedef void * ZMQ_Socket;
 
 /* detect really old ZeroMQ 2.0.x series. */
 #if !defined(ZMQ_RCVMORE)
-#error "Your version of ZeroMQ is too, old."
+#error "Your version of ZeroMQ is too old.  Please upgrade to version 2.1 or to the latest 2.0.x"
 #endif
 
 typedef void * ZMQ_Socket;