Fix scrolling issue

@pfiller [A bug](https://github.com/harvesthq/chosen/issues/1184) was introduced into Chosen from PR https://github.com/harvesthq/chosen/pull/1155. I've solved it by fixing the wheelDelta property as well as allowing it to fail gracefully if it's not present. I've also update our example page to use jQuery 1.7, since that's our minimum standard.