Rails 6 support fo 0.9.x

[grk]

This backports Rails 6 compatibility to the 0.9.x branch.

Because of other dependencies we can't use 0.10 yet, but would like to upgrade to Rails 6.

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
• My submission passes all tests. (Please run the full test suite locally to cut down on noise from travis failures.)
• I've used Github auto-closing keywords in the commit message or the description.
• I've added/updated tests for this change.

New Feature Submissions:

• I've submitted an issue that describes this feature, and received the go ahead from the maintainers.
• My submission includes new tests.
• My submission maintains compliance with JSON:API.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Test Plan:

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

[lgebhardt]

@grk Thanks

[grk]

@lgebhardt thanks for merging this in. Would a release of 0.9.x be possible? Faster than bundling from git.