Update composer install instructions to use require

It's recommended that `composer require` be used for installing dependencies instead of manually altering the `composer.json` file.

Installation steps are now much simpler, only the one require step, instead of requiring people to run `composer update` and `composer install` manually.

This came up in [a discussion](https://github.com/KnpLabs/snappy/pull/118#discussion_r20793023) in another project and I figured it would be good to adjust here as well.