Support hoverable: true in jQuery v1.2.6

jQuery v1.2.6 does not support [`mouseleave`](http://api.jquery.com/mouseleave/).  However [`README.md`](http://daringfireball.net/projects/markdown/syntax#link) states v1.2.6 is supported, even for interactions (although jQuery v1.3.2 is recommended for performance reasons); *You need at least jQuery 1.2.6, but try at least 1.3.2 for interactive charts because of performance improvements in event handling.*

This change prevents a fatal javascript error when using flot with `hoverable: true`.