Fix for Postgres SQL generation error

[lgebhardt]

This fixes an issue reported in Gitter where the generated SQL isn't compatible with Postgres.

ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list

This fix tracks the fields used for sorting and ensures they are in the SQL used to pluck the results. The sort fields are tracking in an option and can therefore be set in a sort's apply callable. The join_manager was also moved into the options in order to allow correct tracking of joins.

I still need to update the documentation for this. I'll try to update this PR with some examples soon.

Most of the code changes are related to the tests and getting Travis testing with postgres. As of now you need to manually create the database (rake won't work) and set the DATABASE_URL environment variable to test locally against postgres. For example:

DATABASE_URL=postgresql://postgres@localhost/jr_test bundle exec rake test

Eventually I'll see if I can load the default rails rake tasks so a db:create can work, but that will be another PR.

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• Maintains compliance with JSON:API
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[CharlieIGG]

LGTM. Can we get this merged 🙏 ?