Fix for composer plugin BC break

[hjr3]

• New composer plugin support breaks backwards compatibility for plugins of
  type 'composer-installer'.

Fixes #51

[till]

Just fyi — this will fail with older versions of composer.

It will look for this package and complain that it cannot find it. Maybe document that in the README because it'll be a WTF moment for many. ;)

[renanbr]

It's a really WTF moment... for a good reason :-)
maybe it's also a good moment to rename the project #37

[hjr3]

they can pin an older version in composer though. I can update the README and let people know to pin to 308220f to make it work with older versions.

[till]

@hjr3 Yeah, that's a good idea.

[hjr3]

I added a note to the README for people using older versions of composer to pin to version 0.0.12. @RobLoach can you take a look?

[RobLoach]

Very nice! Thanks a lot for the help, everyone. It would be great to push Composer Plugin API forwards. Do we have an issue open for that already?

[hjr3]

We may want to actually revert this commit, now that composer fixed the BC break: composer/composer@689e305