Adjust Podfile.

When I ran
$ pod install
got error
[!] Required version (AFNetworking (~> 2.0)) not found for `AFNetworking`.
Available versions: 2.0.0-RC3, 2.0.0-RC2, 2.0.0-RC1, 1.3.2, 1.3.1, 1.3.0, 1.2.1, 1.2.0, 1.1.0, 1.0.1, 1.0, 1.0RC3, 1.0RC2, 1.0RC1, 0.10.1, 0.10.0, 0.9.2, 0.9.1, 0.9.0, 0.7.0, 0.5.1