GPRS::getDateTime: improved timing
GPRS::getDateTime: improved timing

Replaced sim900_read_buffer() by sim900_read_string_until().
This way it takes less time for the method to complete because it does
not need to wait for a timeout. Instead it quits waiting when "OK\r\n"
was received.

Seeed-Studio/GPRS_SIM900,Seeed-Studio/GPRS_SIM900