Update DS3231.ino to use non-deprecated Adafruit_SSD1306 API

[asreimer]

The RTC kit is great! I was able to get it up and running in 1.5 hours with no previous Arduino experience.

I have 2 suggestions for improving the tutorial to make it a bit easier:

1. In the "DS3231.ino" file, replace line 10 with:
   Adafruit_SSD1306 display(SCREEN_WIDTH,SCREEN_HEIGHT,&Wire,OLED_RESET,400000UL,100000UL);
   since this is the way that the Adafruit API for the OLED screen suggests. This line works without requiring the user to modify the Adafruit_SSD1306.h file
2. Remove the instructions about modifying the Adafruit_SSD1306.h file from the tutorial guide.

Thanks!