Update CanCanCan dependency to allow v2+

[grumpit]

This enables projects that use Canard to update to the latest version of CanCanCan and handles deprecation warnings for Rails 6 that are resolved in v2.x

[james2m]

Thanks for the contribution. Happy to merge if you can get the CI passing. I’m traveling right now so won’t get hands to a keyboard for a few days.

[grumpit]

So it has to do with the version of SQLite3 that Rails 3.x wants vs. what's available. If I pin sqlite3 to ~> 1.3.5 in Gemfile, I can get tests to run, but with lots of deprecation warnings, a circular reference warning, and errors. I think much of this has to do with the old version of Rails, but I'll have to do more digging later on.

[grumpit]

All the errors are around Mongo, which no one should be using anyway ;)
EDIT: this was because I don't have Mongo installed locally, so I pushed the modified version, which ran, and passed on Ruby builds for 2.2 - 2.4. Fails for Ruby < 2.2 and jRuby... (see details here)