Modal window, scrollbar shift regression

Current getbootstrap.com:

![Modal shift in IE11](http://tyler.zone/bootstrap-modal-shift-ie11.gif)

This was a problem that was originally solved by 0907244256d923807c3a4e55f4ea606b9558d0ca 

A regression was caused by the removal of the line:

    if (document.body.clientWidth >= window.innerWidth) return 0;

from the `measureScrollbar` function in commit da74fba490673cd4b60e9485b4f0a7aea225d7cc

This seems like a mistake. Further, checking if `document.body.scrollHeight > document.documentElement.clientHeight` (the current scrollbar detection solution) is insufficient to check for the presence of scrollbars on IE10/IE11. IE10/IE11 don't have a scrollbar width since the scrollbar is hidden a majority of the time  because of the CSS `@-ms-viewport { width: device-width; }`

I believe the situation is similar for Chrome on OSX, but I haven't tested that.

Either way, the solution in this commit shouldn't interfere with the goals of da74fba490673cd4b60e9485b4f0a7aea225d7cc as far as I can tell AND fixes an important regression.