Fix for "require" field bug in IE

Hello,

I'd been having some trouble with parsley where it appeared that none of the validation rules were being applied in IE. After a bit of debugging it turns out this is due to the "required" attribute being checked via "prop" rather than "attr" (as IE9(and older) don't consider the required attribute to be a "property", whereas most modern browsers do).

To fix this I've simply switched the prop call to an attr, so that code such as

    <input type="text" name="bla" required/>

Will behave the same way in both IE and HTML5 compliant browsers. Prior to the fix in IE, the required state would not be enforced by parsley.

I suspect this may also be the issue behind https://github.com/guillaumepotier/Parsley.js/issues/348 , as my initial symptoms were much the same.

Thanks,
Carl