Fix TransitionableTransform bug

Considering the following example:

```javascript
    var Engine = require('famous/core/Engine');
    var Surface = require('famous/core/Surface');
    var TransitionableTransform = require('famous/transitions/TransitionableTransform');
    var Modifier = require('famous/core/Modifier');

    var surface = new Surface({
        size: [100, 100],
        properties: {
            background: 'red'
        }
    });

    var tt = new TransitionableTransform();

    var modifier = new Modifier({
        align: [0.5, 0.5],
        origin: [0.5, 0.5],
        transform: tt
    });

    Engine.createContext().add(modifier).add(surface);

    tt.setTranslate([100, 100, 0], {
        duration: 1000
    });

    tt.setRotate([0, 0, Math.PI/2], {
        duration: 1000
    });
```

This resets the surface to its original position after finishing the rotation, producing a undesirable flash effect.
This commit fixes this issue. While recalculating the matrix might be expensive, at least it works. It seems to me that the _final state is being calculated incorrectly.