Truncate cache table before adding index

SQL/mysql.update.sql

@@ -1,6 +1,8 @@
 -- RoundCube Webmail update script for MySQL databases
 -- Updates from version 0.1-beta and 0.1-beta2
 
+TRUNCATE TABLE `messages`;
+
 ALTER TABLE `messages`
   DROP `body`,
   DROP INDEX `cache_key`,