update_readings incorrectly returns old readings, not new readings

[timburke]

https://github.com/iotile/iotile-device/blame/master/src/common/iotile-reports.ts#L347

This results in the lowest and highest IDs for readings being set incorrectly for fixed reports.

See: https://github.com/iotile/iotile-mobile-ionic/issues/1122