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.
Don’t worry. This table just contains computed stats (the number of sentences for each language) so it’s not a big deal. Its content is updated on sentences import by `create_fill_langStats.sql`, and supposed to be updated whenever a sentence is added, removed or its language gets modified. Although this doesn’t work very well #332.