Test against Rails 6.0.0rc1

To get ready for Rails 6.0.0 we can start testing against 6.0.0.rc1

Rails 6.0.0 has some breaking changes that don't allow it to work with older versions of rspec-rails, so we will need to use rspec-rails 4.0. rspec-rails 4.0 is not compatible with older versions of rails.

I also removed the pessimistic version constraints for the Rails libraries in the gemspec. These constraints make it harder for people to upgrade, and it is something Rails itself is moving away from.

The tests are passing on Ruby 2.6. On the Ruby 2.5 build bundler doesn't seem to be able to resolve the dependencies. I will try gem "railties", "~> 6.0.0.rc1" in the Appraisal file instead of loading each library separately to see if that gets what we need. Edit: the build is now passing for Ruby 2.5 as well.

Rails 6 is not compatible with Ruby 2.4 so we will need to either stop running against Ruby 2.4 altogether, or find something similar to Travis's exclude (see the factory_bot_rails travis setup) so we can skip running Rails 6 with Ruby 2.4. I am still not all that comfortable with circleci config, so this might take me some time. Updating to circleci 2.1 and using commands seems to have done the trick.