Fix for issue #68: return consistent values from getSteps(), getClosestStep(), ... when only one slide is available

This fixed the problem, but wouldn't it be better if we fixed it from the root and make the `calculateStepRatios` function sane? E.g.

```js
calculateStepRatios: function() {
  var stepRatios = [];
  if (this.options.steps >= 1) {
    for (var i = 0; i <= this.options.steps - 1; i++) {
      if (this.options.steps > 1) {
        stepRatios[i] = i / (this.options.steps - 1);
      } else {
        // A single step will always have a 0 value
        stepRatios[i] = 0;
      }
    }
  }
  return stepRatios;
},
```