JavaScript strict mode compatibility

This pull-request may be nitpicky, but I noticed that strict mode would return an error when modernizr was set to use it.

The problem was the mod function was being passed 'this' as the window parameter which, [per the specification of strict mode (page 235)](http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf):

> If this is evaluated within strict mode code, then the this value is not coerced to an object. A this value of null or undefined is not converted to the global object and primitive values are not converted to wrapper objects. The this value passed via a function call (including calls made using Function.prototype.apply and Function.prototype.call) do not coerce the passed this value to an object (10.4.3, 11.1.1, 15.3.4.3, 15.3.4.4).

This quick change fixes this.