Newer chips revision default address is 0x6A and the new answer for the register LSM6DS3_ACC_GYRO_WHO_AM_I_REG is 0x6A.
So that the begin function won't return success.
Line 107 of SparkFunLSM6DS3.cpp should be updated to below, to reflect the new answer of the above register
if (( readCheck != 0x69 ) && ( readCheck != 0x6A ))