Update composer installation instructions to reference `wp-cli/wp-cli-bundle`.

[Timothy Jacobs]

The current docs recommend installing composer globally like so:

composer global require wp-cli/wp-cli

It seems like that should be replaced with

composer global require wp-cli/wp-cli-bundle

https://make.wordpress.org/cli/handbook/installing/#installing-via-composer

[Kimberly Cottrell]

I've gone ahead and edited this section of the documentation - #279.

Composer was tested on a PHP 7.2 Docker container with debian-slim as the underlying OS. Windows testing still needs to happen.

[Alain Schlesser]

Fixed via #279