If you just want the ceiling:

guiText.text = prefix + Mathf.Ceil(timeRemaining) + suffix;

If you just want to drop the decimal and get the floor:

guiText.text = prefix + Mathf.Floor(timeRemaining) + suffix;

If you want to truncate the time, you could have Unity do the timing for you and then just print that.

var time: int = 60;

function Awake () {
    //Create the timer.
    //Method: InvokeRepeating("MethodName", delayFor1stFire : float, repeatTime: float);
    InvokeRepeating("FireTimer", 1, 1);

    //I could not figure out why you were calling these every frame.
    transform.position.x = 0.65;
    transform.position.y = 0.05;
}

function FireTimer() {
    time--;

    guiText.text = prefix + time.ToString() + suffix;

    if(timeRemaining <= 0)
    {
         Application.LoadLevel ("Game Over");
    }
}

This will improve performance as well because you are not calling the method every frame. Only once per second. So, if performance matters, this is the fastest method.

guiText.text = prefix + timeRemaining.ToString("f0") + suffix;

This should work.

You could try Mathf.Round() or something similar, or just convert the float to an int it will drop the fraction.

var intTimeRemaining : int = timeRemaining;
guiText.text = prefix + intTimeRemaining + suffix;

Unity JS has dynamic typecasting, so it should just work. I didn't test it, though.

Cheers

==

You can use an integer and subtract one every second, instead of using a float in Update. See my answer here: http://answers.unity3d.com/questions/12992/count-down-timer-per-second