Update Bullet gem to 6.0.2

What does this MR do?

We are having a problem with the Bullet gem:

An error occurred while loading ./spec/requests/api/users_spec.rb.
Failure/Error: require File.expand_path('../config/environment', __dir__)

Bundler::GemRequireError:
There was an error while trying to load the gem 'bullet'.
Gem Load Error is: Bullet does not support active_record
ActiveRecord::VERSION yet

This update also adds support for Rails 5.2 and Rails 6.0

See the changelog: https://github.com/flyerhzm/bullet/blob/master/CHANGELOG.md

The complete changes between versions: https://github.com/flyerhzm/bullet/compare/5.5.0..6.0.2