Fix Rails 5.2 incompatibility

[flackou]

Following my comment on PR #3, I propose this change to fix the error raised on app loading with Rails 5.2 :

NoMethodError: undefined method `sqlite3=' for ActiveRecord::Base:Class
.../rails_wordpress/lib/rails_wordpress/railtie.rb:9

My opinion is that the gem should not change the belongs_to_required_by_default config of its parent application. In my company, we use rails_wordpress as a Rails engine in a larger app, with another database and other models living aside. They should not be concerned by this setting. So I suggest to replace this global config by individual optional: true on every belongs_to association declaration (as suggested by @brijeshror himself in his initial PR #3). I added a test on creating a Post without parent.

I also update the Rails locked dependency (used in tests) to latest 5.2 version.

[SpiffyStores]

It would be good if this could get sorted out as we are trying to use this under Rails 5.2 as well.