Bump version to 3.1.6

NEWS

@@ -1,3 +1,9 @@
+libmodbus 3.1.6 (2019-07-31)
+============================
+
+- Fix awful typo in fix for VD-1301 vulnerability.
+  Thank you @karlp.
+
 libmodbus 3.1.5 (2019-07-29)
 ============================
 

configure.ac

@@ -13,7 +13,7 @@
 #
 m4_define([libmodbus_version_major], [3])
 m4_define([libmodbus_version_minor], [1])
-m4_define([libmodbus_version_micro], [5])
+m4_define([libmodbus_version_micro], [6])
 
 m4_define([libmodbus_release_status],
     [m4_if(m4_eval(libmodbus_version_minor % 2), [1], [snapshot], [release])])