Suggest, not require Magento Composer Installer

Additionally removed repositories section as it is not needed any longer.

Refrences:

- https://github.com/magento-hackathon/magento-composer-installer/blob/master/doc/FAQ.md
- https://getcomposer.org/doc/04-schema.md#suggest
Merge pull request #31 from tkdb/patch-1

Suggest, not require Magento Composer Installer