spec: tweak nested_instafail_formatter for rails3

Change-Id: I392a001f8863ccd9511a03a39b06e38a19288971
Reviewed-on: https://gerrit.instructure.com/28816
Tested-by: Shawn Meredith <shawn@instructure.com>
QA-Review: Shawn Meredith <shawn@instructure.com>
Product-Review: Shawn Meredith <shawn@instructure.com>
Reviewed-by: Shawn Meredith <shawn@instructure.com>