Update Podspec to support Cocoapods 0.38.x

[levi]

Fixes #659

[nguyenhuy]

Hm, Travis is still using CocoaPods 0.37.2 because it is explicitly specified here. You may want to change that as well.

[appleguy]

I'm a little intimidated by this Cocoapods version because it broke so much stuff...Not just ASDK, but also Pinterest. I appreciate you looking into it, though, because it has to be done eventually. Once Travis is succeeding, I'll hit the button and pray in several languages.

[levi]

@appleguy the same thing happened to us a while ago. We recently upgraded to 0.38.2 and seem to be running fine. I just updated travis—let's see what happens.

[appleguy]

Thanks for forging the path. I will now promptly flee my computer and pretend success is inevitable, as is not too hard to do with such competent help from folks like you :).