Poke migration-management versions for 1.6.4

Author: jcflack

pljava/src/main/java/org/postgresql/pljava/internal/InstallHelper.java

@@ -889,6 +889,7 @@ void migrateFrom( SchemaVariant sv, Connection c, Statement s)
 		UNREL20040120  ("5e4131738cd095b7ff6367d64f809f6cec6a7ba7"),
 		EMPTY          (null);
 
+		static final SchemaVariant REL_1_6_4       = REL_1_6_0;
 		static final SchemaVariant REL_1_6_3       = REL_1_6_0;
 		static final SchemaVariant REL_1_6_2       = REL_1_6_0;
 		static final SchemaVariant REL_1_6_1       = REL_1_6_0;