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.
Merge branch 'master' of http://github.com/Tatoeba/tatoeba2
intermediate commit for issue #234 (not working yet)
#234 Set default values for $showTranslationsInto and $notTranslatedInto
#234 Moved the cookie stuff into beforeRender()
#234 Changed default value for TATOEBA_DOMAIN
Revert "database/updates: Add command to drop table 'languages' if it exists"

This reverts commit f6f2646a00218d4cab9132a07f57fc9784e3c7de.
Merge pull request #340 from Tatoeba/lang_cookie

Pull request for #234