Fix event.returnValue warnings in Chrome

No need to worry about adding more code.  Closure compiler ends up
rewriting this as

```
(b.preventDefault?b.preventDefault():b.returnValue=!1,
b.stopPropagation?b.stopPropagation():b.cancelBubble=!0)
```
Increase version to 1.4.6
Minimize
Merge github.com:ccampbell/mousetrap