Problem: new public symbols added

Solution: bump ABI version fromo 5:0:0 to 6:0:1 since 4.2 is backward
compatible with 4.1, but new symbols are there

Author: bluca

configure.ac

@@ -37,9 +37,10 @@ AC_SUBST(PACKAGE_VERSION)
 # ZeroMQ version 3.1: 3:0:0 (ABI version 3)
 # ZeroMQ version 4.0: 4:0:0 (ABI version 4)
 # ZeroMQ version 4.1: 5:0:0 (ABI version 5)
+# ZeroMQ version 4.2.0: 6:0:1 (ABI version 5)
 #
 # libzmq -version-info current:revision:age
-LTVER="5:0:0"
+LTVER="6:0:1"
 AC_SUBST(LTVER)
 
 # Take a copy of original flags