Do not replace packages

Don't ever replace packages you don't own. It will cause composer
to fall back to *your* package if it cannot find a suitable
version of the original one.

We've had numerous breakages due to this, where random people
were replacing zendframework or symfony components and thus
borked the internet.

Side note: Yes, packagist really should prevent you from doing
stupid things like this as it is non-obvious. Unfortunately it
currently does not.