Add sensiolabs/security-checker

[madnight]

This tool detects vulnerabilities in composer dependencies https://github.com/sensiolabs/security-checker

[jakzal]

Sounds great, do you wanna send a PR?

[madnight]

I'm a bit lazy, but if you insist i would

[jakzal]

No worries, it's no biggie for me. I can do it.

[madnight]

#8