Fix composer.json

There were so many issues, I don't even know where to begin.

* Lock file was referencing a symfony 2.1 RC version that was deleted from github.

* composer.json was referencing composer/composer which depends on newer versions of symfony components. incompatible with the 2.1.* constraint it was using.

* Everything using untagged releases.

* Lots of unneeded dependencies from the bloated "fat" variant of silex.

I hope I didn't break anything. It did work on my machine (tm), but make sure you test it properly before deploy...