Bump version to 1.7.0pre

(This should avoid folks thinking master represents 1.6.0.)
The next release needs to bump the minor because of the new
option NNG_OPT_REQ_RESENDTICK.

include/nng/nng.h

@@ -57,9 +57,9 @@ extern "C" {
 // We use SemVer, and these versions are about the API, and
 // may not necessarily match the ABI versions.
 #define NNG_MAJOR_VERSION 1
-#define NNG_MINOR_VERSION 6
+#define NNG_MINOR_VERSION 7
 #define NNG_PATCH_VERSION 0
-#define NNG_RELEASE_SUFFIX "" // if non-empty (i.e. "pre"), this is a pre-release
+#define NNG_RELEASE_SUFFIX "pre" // if non-empty (i.e. "pre"), this is a pre-release
 
 // Maximum length of a socket address. This includes the terminating NUL.
 // This limit is built into other implementations, so do not change it.