fixed loop timing

[Bingdom]

time.time() is susceptible to ntp adjustments. Use .monotonic() to avoid it.

In the rare case where the OS adjusts the time forward while waiting for conversion, it will raise ADS1015TimeoutError.

This might explain the occasional ADS1015TimeoutError exception I get. Source: #63

[Bingdom]

@Gadgetoid Do you want me to update the changelog and increment the version number?