demo在在iOS9运行正常，但在iOS8.4中运行报错。
报错详情：Warning: skipping file '/Applications/Xcode 2.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.4.sdk/usr/lib/libz.tbd' (unexpected file type 'file' in Frameworks & Libraries build phase)
Warning: skipping file '/Applications/Xcode 2.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator8.4.sdk/usr/lib/libz.tbd' (unexpected file type 'file' in Frameworks & Libraries build phase)
原因是iOS8.4中没有libz.tbd 和libstdc++.6.0.9.tbd两个库，
解决办法，导入libz.dylib 和 libstdc++.6.0.9.dylib即可。