[FIX] filtering out test images in podspec/prepare_command
Merge pull request #671 from imihaly/Issue669

Fix prepare_command in podspec
Facebook iOS SDK 3.21
Facebook iOS SDK 3.21

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

Upgrading:
    https://developers.facebook.com/docs/ios/upgrading/