rails3: explicitly include ERB::Util in helper specs

rails3 doesn't implicitly include ERB::Util anymore for specs, thus we don't
have access to h() and friends unless we explicitly include it

Change-Id: I58f2009d0d92128a9e6c67ab5b25c6c8de8b19d1
Reviewed-on: https://gerrit.instructure.com/30964
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>