Fix AR adapter when table doesn't exist

[jnunemaker]

This only happens when you haven't migrated yet likely.

xref #557 (comment)

[bkeepers]

🙇 thanks for fixing

[granth]

This also happens during db:setup in a CI run. Not a big deal!

[bkeepers]

@granth can you confirm this is fixed in 1.1.2?

[granth]

Sorry, I should have been more clear. I see the flipper_gates warning in 1.1.2 during db:setup.

Mysql2::Error: Table 'circle_test.flipper_gates' doesn't exist. You likely need to run `rails g flipper:active_record` and/or `rails db:migrate`.
Already memoized items
Running via Spring preloader in process 871
** Invoke db:setup (first_time)

I mean, it's true that the table doesn't exist!

Update: looks like this is triggered by $flipper = Flipper.instance in an initializer, which is there for historical reasons.