Fixed code so that GMF now works with IMA SDK beta 7

Fixes issue #49 
I think objective C style prefers var names like "overlayVc" instead of "VC" but I could be mistaken. Perhaps check if there's a preference or see if there are other vars you can copy from for this?

Also, this line should be:
[overlayVC setISAdDIsplayed:NO];

"Dot notation is allowed only for access to a declared @property."
http://google-styleguide.googlecode.com/svn/trunk/objcguide.xml?showone=Properties#Properties