Use "prepend-autoloader": false

Composer's autoloader is slow, taking about 63µs per class, whereas
MediaWiki's is fast, taking about 7µs per class. This adds up to an
overhead of about 13% of CPU time on short requests such as API calls.
On those same short requests, MediaWiki's autoloader finds most of the
classes. So it makes sense to run it first, before the two Composer
autoloaders. So change the config so that composer passes
$prepend=false to spl_autoload_register().
Merge pull request #32 from tstarling/prepend-autoloader

Use "prepend-autoloader": false