spec: vendor_gems test.sh result determination tweak

Change-Id: I68ca527f6e48e0305315d824aaf9a9625ecd8276
Reviewed-on: https://gerrit.instructure.com/30675
Reviewed-by: Bryan Madsen <bryan@instructure.com>
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>