remove AR dependency from PaginatedCollection gem

fixes: CNVS-12271

test plan:
- no test needed, spec changes only

Change-Id: I54a814319e17f8ca05c5b8652fc13db70258ff5a
Reviewed-on: https://gerrit.instructure.com/33515
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>