Use correct SQLite3Adapter superclass in Rails 4

Hi there.

I'm using Database Cleaner in a Rails 4 (edge) project, and the current master fails with:

.../database_cleaner-0.9.1/lib/database_cleaner/active_record/truncation.rb:200:in `<module:ConnectionAdapters>': superclass mismatch for class SQLite3Adapter (TypeError)

Looks like the SQLite3Adapter superclass changed in Rails 4. This commit checks the superclass in truncation.rb and should preserver Rails 3 compatibility.