Facebook iOS SDK 3.18

Summary:
    see https://developers.facebook.com/ios/change-log-3.x/

Upgrading:
    https://developers.facebook.com/docs/ios/upgrading/
Merge commit '331e7d38de3b2dbe4f6471f86be18d00aae2c740'

* commit '331e7d38de3b2dbe4f6471f86be18d00aae2c740':
  Facebook iOS SDK 3.18