Added a check for insecure composer packages

Aloha, i thought it might be a good idea to add one more check.

This one will run the current projects composer.lock file against https://security.sensiolabs.org/ checking if the composer packages have any known vulnerabilities.

documentation has been added to the readme.

I've also added this check as one of the requirements in RMT's own .rmt.yml, as it is a good idea to enable it.

Also this pullrequest contains an updated lock file, whoever did edit the composer.json last forgot to run composer update afterwards. Since i added a new dependency i had to run update.

regards,
PP