Replace composer.json

[k1sul1]

As plugins and themes are managed by Composer, they have to be tracked in composer.json.

Seravo/wordpress has one already though. It installs plugins that we might not want, and doesn't install plugins that we might want.

Replace it with Composer post-install-cmd or in vagrant up.

I've no knowledge about whether or not you can prompt in scripts that run in post-install-cmd, and replacing a core file isn't something that I want to do without asking the user, so vagrant up might be a better place.