Avoid using lease_connection

[joshuay03]

Follow-up to d1eade0

Ideally, using ActiveRecord::ConnectionHandling#lease_connection usage should be avoided unless absolutely needed:

IMO calling .lease_connection in a place other than a script or rake task is a bug and should be reported to whatever gem does it.

We don't really need a connection here since we just need to figure out the adapter, which we can grab from ActiveRecord::DatabaseConfigurations::HashConfig#adapter which is already down-cased.

[joshuay03]

While I'm here, I recommend expanding your CI matrix to include all Rails versions you're willing to explicitly support, as well as other adapters (at least trilogy and postgresql). FWIW I ran tests locally with postgresql and all but one passed.

[charkost]

Thanks @joshuay03 !