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 * out-of-date source repos which you can update with `pod repo update` or with `pod install --repo-update`.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。