fix: add railtie to clear caches when code reloads

[adamkiczula]

fixes #1439

This adds a railtie hook to clean up the JSONAPI::Resource caches when code reloads so that we don't have stale references to classes lying around.
I'm not sure how to test this one in a test suite, but there's a reproduction script in #1439 that passes with this.

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. Class caching is too aggressive in dev #1439
• 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.

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. (see script in Class caching is too aggressive in dev #1439, happy to incorporate into CI if you have ideas on how to do that)

Test Plan:

Reviewer Checklist:

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

[bf4]

💯

[bf4]

noting that this is related to the same issue we were trying to address in #1440 and #1434

[lgebhardt]

Thanks @adamkiczula!