As described in #1357 (comment), we need to support restricting updates according to the platform version, e.g. php <= 5.6.

Because our lookups are done independently of composer, we need to reproduce the logic that composer uses to determine if a newer version is compatible.

Steps:

1. When parsing the composer.json file, determine if it includes a php field under platform

2. Pass the php restriction to the packagist datasource lookup

3. Packagist datasource should return only compatible versions