Make sure imu.begin is NOT commented out. The gist version is OK but the version in the github repo has commented out imu.begin. See the code below. The Sparkfun Thing defaults to pins 2,14 for I2C so it should work without changes. But it does not hurt to specify the pins.
/*
//------------------------------------------------------
// The above lines will only take effect AFTER calling imu.begin(), which verifies
// communication with the IMU and turns it on.
if (!imu.begin())
{
Serial.println("Failed to communicate with LSM9DS1.");
while (1)
;
}
*/