Rename kAFOAuthCredentialServiceName to kAFOAuth2CredentialServiceName

Fixes a duplicate symbol error when linking to AFOAuth1Client.

Ref https://github.com/AFNetworking/AFOAuth1Client/blob/master/AFOAuth1Client/AFOAuth1Client.m#L135

Also see https://github.com/AFNetworking/AFOAuth1Client/pull/81