pod install报错：
[!] CocoaPods could not find compatible versions for pod "xxx":
In snapshot (Podfile.lock):
Analyzing dependencies
[!] CocoaPods could not find compatible versions for pod "SDWebImage":
In snapshot (Podfile.lock):
SDWebImage (= 5.1.0)
In Podfile:
SDWebImage
None of your spec sources contain a spec satisfying the dependencies: `SDWebImage, SDWebImage (= 5.1.0)`.
You have either:
* out-of-date source repos which you can update with `pod repo update` or with `pod install --repo-update`.
* mistyped the name or version.
* not added the source repo that hosts the Podspec to your Podfile.
Note: as of CocoaPods 1.0, `pod repo update` does not happen on `pod install` by default.
解决办法：.xcworkspace和Podfile.lock文件删除，重新pod install。
