Enable Composer autoloading

Per https://www.mediawiki.org/wiki/Manual:Extension_registration:

If an extension or skin has library dependencies, it may have
a composer.json file as well, see Manual:Composer.json best practices.
Use the load_composer_autoloader field to make MediaWiki use
Composer's autoloading when appropriate.

Among other things, this makes the extension work correctly with
MediaWiki-Vagrant.

Change-Id: I8237825881b7ce12707cc754a34e96b7f3dad8dc