Handle millis overflow during running state

[boraozgen]

If millis overflowed during running state, now < time_start_ always returned true and hanged the library. This change uses the signed integer trick to handle the overflow situation.

For this to work, millis must overflow at 32-bits. I updated the Mbed OS millis implementation to use the latest version to ensure this.

[jandelgado]

Hi, thanks for the fix. Could you also add a unit test that checks the behaviour?

For the curious, some background: https://arduino.stackexchange.com/questions/12587/how-can-i-handle-the-millis-rollover/12588#12588

[boraozgen]

Done, I also found another problematic line while writing the tests (L.402) and fixed it too.

[coveralls]

Coverage remained the same at 96.875% when pulling b448e76 on boraozgen:handle-millis-overflow into 6ebb7fc on jandelgado:master.