added two composer related checks to prerequesites

Ahoy,

i've added two prerequesite checks, to allow to check for issues that I've seen in day to day life,

One of them runs composer validate on the composer.json, as there is no reason for a release with a broken composer json, and we can get the manual labor of people running composer validate automated away from them here.

The second one actually allows to have RMT check if the composer.json is set to a specific minimum-stability,
I have found that when you are working on a project with several packages in work it makes a better workflow to go with dev-master (or similar) dependencies, however once you release a version, you really want to have stable dependencies.

regards,
PP