Update PromiseKit.framework build setting to indicate that its content contains Swift. Otherwise it will cause a runtime error on start up:
dyld: Library not loaded: @rpath/libswiftAssetsLibrary.dylib
  Referenced from: /Users/Allen/Library/Developer/Xcode/DerivedData/Tappl-aqulklvsimmnwtfpadxznzhxpxuy/Build/Products/Debug-iphonesimulator/PromiseKit.framework/PromiseKit
  Reason: image not found