Changed _next_read_ms to _last_read_ms to prevent millis() rollover

[gutierrezps]

Hi. Upon inspection of the source code, I saw you were checking if the tag was still near by setting _next_read_ms to a future value. This is not a good approach since millis() overflows every 50 days. The appropriate approach is to mark a previous millis and compare with the current value, as shown on this StackOverflow answer and also used on the BlinkWithoutDelay Arduino example.

So what I did was change _next_read_ms to _last_read_ms and update is_tag_near() to see if millis() - _last_read_ms < RDM6300_NEXT_READ_MS.

[arduino12]

Thanks @gutierrezps, I just learned something new thanks to you!