spec: once-ler spec/models/[r-w]*

in rails3, reduce runtime >50% (8:23 -> 4:04 locally)

Change-Id: Ie5a81e2088a4fe579f3ce1d04345743ccd46458a
Reviewed-on: https://gerrit.instructure.com/37311
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>