Update composer.lock with wordpress 3.9

At the moment 'composer create-project' breaks (with an error) on bedrock because the composer.lock isn't updated accordingly to composer.json. I think this solves it. All I did was to fork it, clone it, run composer update wordpress/wordpress (which also did some updates for composer/installer). Might be confusing to new folks with the error, so I thought I'd give it a try and that this would be a reasonable pull request! :)

Untested since I don't have a project on composer to simulate it all the way!!!

Happy easter from Sweden!