[composer] Remove useless/harmful fields, fix autoload definition

* Version will give you problems when you want to actually tag a
  release and the versions don't match up.

* Repositories do not to what you think they do. See also the composer
  docs.

* The autoload definition was broken. See also composer docs, PSR-0
  spec.

This PR was triggered by this StackOverflow question:
http://stackoverflow.com/q/15451371