Add compatibility for Rails 6.0 and remove support for Rails 4.2

[alecclarke]

To add Rails 6.0 compatibility for polymorphic_integer_type, we need to:

• Bump the activerecord gemspec dependency range to include 6.0
• Add a gemfile/tests for Rails 6-0-stable
• Update the call to ActiveRecord::MigrationContext's initializer as the arguments have changed in 6.0
• Remove calls to update_attributes in the specs as update_attributes is deprecated in Rails 6.1 (not required, but might as well handle it here)

On top of this, since Rails 4.2 is no longer supported by Rails, I'm removing 4.2 support for all subsequent versions of polymorphic_integer_type.

Note: While we don't have automated tests yet, running rake test:all passed locally.