Fix bug not re-calculating scrollbar width when opening modal

If opening a modal when a scrollbar is present, closing it, then resizing the window so the scrollbar disappears, then opening the modal again, the content would shift. This fixes the problem by always calculating the scrollbar width every time a modal is opened.

See buggy example: http://jsfiddle.net/02rthxw5/
See fixed example: http://jsfiddle.net/02rthxw5/2/

Should I add compiled files to the PR? I could not find if this is desired in the docs.
Removing this whole method seems like a breaking change to me. Maybe just rewrite it to the following instead of removing it altogether.
```js
Modal.prototype.checkScrollbar = function () {
  this.scrollbarWidth = this.measureScrollbar()
}
```