Mention CanCanCan in Priceless Gems section

The original [CanCan][1] gem became less active. The last commit was 7 months
ago. A community is slowly switching to [CanCanCan][2] gem. So I think it is
worth to be mentioned.

[1]: https://github.com/ryanb/cancan
[2]: https://github.com/CanCanCommunity/cancancan