improved podspec, it should support OSX

Thanks for the awesome wrapper!
I've just used it on OSX, and after manually fixing the build, figured out it's fixed in 2.1.

My project is unit tested, so encryption should be properly working in OSX 10.8.

Currently, .podspec points to an older tag (2.0).
With this patch, you don't have to update on 2 places anymore, just keep the convention.