Parser-inserted `script` elements will be executed asynchronously whether the async attribute is set or not. Unless we want full support for Firefox 3.6 and OmniWeb 622 (which need this line).

Spec says:
"If was-parser-inserted is true and the element does not have an async attribute, then set the element's "force-async" flag to true."
http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting-1.html

More on that in Steve Souders' research: http://www.stevesouders.com/blog/2012/01/13/javascript-performance/