Update Bullet gem to 6.0.2

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