From personal experience on my IoT project with the arduino dock 2 and I2C - leave A4 and A5 open circuit on the dock - weird yes but if you add any I/O to them the I2C stops dead in it's tracks. Once you get it working pay attention to the I2C buffer in the arduino - clear it often as it fills up quick and starts responding 0xff to any and every query. Articles like this one helped me a lot: