spec: once-ler spec/models/[d-g]*

in rails3, reduce runtime by 45% (11min -> 6min locally)

Change-Id: Id7253cb71a7f96a4f3fd4a979cbdea635edec35f
Reviewed-on: https://gerrit.instructure.com/37115
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>