Facebook iOS SDK 3.17.1

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

Upgrading:
    https://developers.facebook.com/docs/ios/upgrading/
Merge commit 'c7159944bcf1d5f72db483fc7188d6983d74a280'

* commit 'c7159944bcf1d5f72db483fc7188d6983d74a280':
  Facebook iOS SDK 3.17.1