Bump ABI version for 0.6-beta.
diff --git a/src/nn.h b/src/nn.h
index 0271c03..4fa42c5 100644
--- a/src/nn.h
+++ b/src/nn.h
@@ -66,7 +66,7 @@
 #define NN_VERSION_CURRENT 2
 
 /*  The latest revision of the current interface. */
-#define NN_VERSION_REVISION 1
+#define NN_VERSION_REVISION 2
 
 /*  How many past interface versions are still supported. */
 #define NN_VERSION_AGE 2