[BUGFIX beta] Force remove `required` attribute for IE8

In IE8, `elem.prop('required', '')` couldn't remove attribute.
(This problem is occured at `required` only. Other attributes work as our expectation.)

``` javascript
view.set('required', false);
view.$().get(0).outerHTML; // this includes `required` attribute.
```

To remove attribute, `removeProp` is needed.

This commit fixes this failure:
![2014-05-27 0 00 09](https://cloud.githubusercontent.com/assets/290782/3083828/728121a0-e4e9-11e3-94ce-1c3de65adc4b.png)

I think this change may have far-reaching ramification.
So I tested the following browsers and confirmed working fine:
* FireFox 28.0
* Safari 7.0.4
* Google Chrome 36.0.1985.18 beta
* Internet Explorer 8