Use the correct gemspec so SQLite3 gets pulled in with bundle install

This ensures SQLite3 will be pulled in on the Travis builds for testing against edge Rails. 

It will still fail, for a different reason:

https://github.com/rails/rails/commit/a1bb6c8b06db83546179175b9b2dde7912c86f9b changes `:uniq` to `:distinct`.

I am not sure whether the correct fix is to change the tests to use `:distinct` instead, or if we need to change `ActiveRecord::Relation::VALUE_METHODS` to include `:uniq`

If you let me know the correct fix, I will include it as well.