rails3: fix quiz submission questions api spec failures

rails3 changed BigDecimal json serialization to return a string, but this breaks
bw-compat with our existing apis. rails4 provides an option to opt-out of this,
so opt-out if we can, but otherwise monkey-patch rails3 to use native behavior

Change-Id: I50796ed1d2d33aa1ab9fce576016b5b08b24b30c
Reviewed-on: https://gerrit.instructure.com/30858
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>
QA-Review: Derek DeVries <ddevries@instructure.com>