Added "return true" to begin function.

[andyfsimon]

As the "begin" function is reporting to return a bool, I expected that I could use it in a similar fashion:

if (!sensor.begin()) {
    Serial.println("Could not find a valid Si7021 sensor, check wiring!");
}

But as the function returns nothing so far, this fails. Added "return true" in two points of the function where it actually finds a sensor.