The CocoaPods master spec got all sorts of messed up a little over a week ago. For that reason you're going to have to delete your local copy and then follow their instructions to set it up again.
The link to their blog that discusses the issue: http://blog.cocoapods.org/Repairing-Our-Broken-Specs-Repository/
The command for getting back up and running:
sudo rm -fr ~/.cocoapods/repos/master
pod setup