Add support for Mongoid 7.0 and 7.1

[joe1chen]

This pull adds support for Mongoid 7.0 and 7.1. Mongoid 7+ had some major changes to the internal classes for relations (now called associations).

Summary of changes:

• Updated references to internal classes for associations.
• Mongoid 7 is a little stricter when it comes to defining associations, so specs had to be updated to add inverse_of in order to avoid Mongoid 7 runtime errors.
• spec_helper.rb had a bug where the association fields nearest_id, nearest_type, etc. were being incorrectly removed, causing a runtime error when running specs in Mongoid 7. The field reset code had to be updated to properly restore fields between runs, and references to fields were also updated accordingly.
• Defining a has_many association with inverse_of: nil behaves differently under Mongoid 7. Prior to Mongoid 7, this association has a nil inverse value, but under Mongoid 7, the association is assigned to a default inverse value. The spec in macros_spec.rb has been updated to pass under Mongoid 7 and to note this difference.

[joe1chen]

By the way this should fix issue #55

[joe1chen]

One more thing to note, I changed the Travis matrix to add Ruby 2.6 and remove Ruby 2.3 since Rails 6 supports only Ruby 2.5+

[joshed-io]

Thanks @joe1chen! Really appreciate the contribution here.