Update vungle.js

Koray, I rewrote this function in another idiom so that it actually returns a an instance of Vungle. Your's didn't. I put in in a slightly different place on the window than you.  You might want to check why your version doesn't seem to.  I tested quite a bit and that was definitely some problem with your function. 

Let me know what you find out.

I am super grateful for all the work you saved me. 

Thanks!

Robert