fix carthage issue

[hongxinhope]

The playgrounds should not be referred in project, otherwise it will cause an Carthage update error as follows:

Build Failed
	Task failed with exit code 65:
	/usr/bin/xcrun xcodebuild -project /Users/*****/*****/*****/Carthage/Checkouts/SwiftPing/SwiftPing.xcodeproj 
-scheme SwiftPing-iOS -configuration Release -derivedDataPath 
/Users/*****/Library/Caches/org.carthage.CarthageKit/DerivedData/8.3.3_8E3004b/SwiftPing/1.1 
-sdk iphoneos ONLY_ACTIVE_ARCH=NO BITCODE_GENERATION_MODE=bitcode 
CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY= CARTHAGE=YES build
 (launched in /Users/******/******/******/Carthage/Checkouts/SwiftPing)

This usually indicates that project itself failed to compile. 
Please check the xcodebuild log for more details: 
/var/folders/3p/4zfv0tq137j6c92rx66cffyh0000gn/T/carthage-xcodebuild.HHmV1u.log