Fix argument error in <= IE9

Was getting an argument error in <= IE9 until I change the call a bit to check for a function first.  Looked like fn was actually 'true' instead of a function on first call...  Weirdly enough I am not getting the same error on the swipejs home page but the slideshow doesn't actually finish going through all the slides, possibly another issue with <= IE9?  See attached.
![screen shot 2013-12-09 at 3 20 11 pm](https://f.cloud.github.com/assets/817344/1710049/5e7aacd8-6129-11e3-9702-71a3ed826ab3.png)