[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.
Merge pull request #4936 from tricknotes/fix-required-for-ie8

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