spec: once-ler spec/helper

in rails3, reduce runtime by >50% (0:43 -> 0:20 locally)

Change-Id: I4fb9bcb346e688606107eb7af0f17e2ab28110ea
Reviewed-on: https://gerrit.instructure.com/37606
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>