Added support for IPv4 in CIDR notation and updated list of allowed IPs

Hello,

I added support for IPv4 in CIDR notation, because currently GitHub uses 192.30.252.0/22. See https://help.github.com/articles/what-ip-addresses-does-github-use-that-i-should-whitelist for more details. Also I updated list of allowed IPs.

Could You merge my changes with your code?

Regards
Piotr Minkina