Version changed to 3.1.0

This version downgrade is done because of the previous refatoring.
It removed all the new features and reintroduced some of the old
ones (identities). Thus, it made trunk much closer to existing 3.0
branch than it used to be.

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>

include/zmq.h

@@ -55,8 +55,8 @@ extern "C" {
 /******************************************************************************/
 
 /*  Version macros for compile-time API version detection                     */
-#define ZMQ_VERSION_MAJOR 4
-#define ZMQ_VERSION_MINOR 0
+#define ZMQ_VERSION_MAJOR 3
+#define ZMQ_VERSION_MINOR 1
 #define ZMQ_VERSION_PATCH 0
 
 #define ZMQ_MAKE_VERSION(major, minor, patch) \