spec: vendored_gems test build

Change-Id: If7abd9e758c54f0d29b1da86ccdf4f076a4219bf
Reviewed-on: https://gerrit.instructure.com/32785
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Reviewed-by: Shawn Meredith <shawn@instructure.com>
QA-Review: Shawn Meredith <shawn@instructure.com>
Product-Review: Shawn Meredith <shawn@instructure.com>
Tested-by: Shawn Meredith <shawn@instructure.com>