database/updates: Add command to drop table 'languages' if it exists

In update 2014-05-25.sql the first command tries to rename table
langStats to languages. But if the update scripts are run on an
existing database, it throws an error that stating table languages
already exists. This fix prevents it by dropping table languages
if it exists.
I'm a little worried about what would happen if this were applied to a database that already had undergone several updates and had a languages table with real content. Perhaps if languages already exists, we should gracefully exit instead.