Old Libraries Being Used

The pod install process doesn't bother telling you if it is selecting old versions of libraries due to compatability with the version of iOS you are targeting. Its easy to assume that when you run pod install you get the latest version of everything, but in fact you get the latest version of each library which is compatible with the platform version you are targeting. When pod install runs it tells you which version of a library it is installing but not if there is a newer version. You can force it to use the version you want by specifying the version number like this:

pod 'BlocksKit',      '~> 1.8.3'

This will generate an error message on pod install if the library can't be used with theplatform being targetted. 

Specify the ios version you are targetting like this:

platform :ios, '5.1'

Re-Install A Library

If you get error messages for libraries during build you can't explain it can be worth deleting the library folder in your pods folder and runnign pod isntall again to re-install it.  This can sometimes fix these sorts of issues if the library has been updated but cocoapods hasn't updated correctly for some reason.