内容提要：
podfile升级之后到最新版本，pod里的内容必须明确指出所用第三方库的target，否则会出现The dependency `` is not used in any concrete target这样的错误。
以下从三个步骤陈述：
1. Podfile升级
查看pod版本：
pod --version
pod 升级（此时是升级到1.0.0.beta.2版本）：
sudo gem install cocoapods --pre
2. 错误内容：
我的Podfile的内容是：
platform :ios, '7.0' pod 'ReactiveCocoa', '2.1.8' pod 'objectiveflickr', '2.0.4' pod 'LinqToObjectiveC', '2.0.0' pod 'SDWebImage', '3.6'
pod install后出现错误The dependency `` is not used in any concrete target
pod install /System/Library/Frameworks/Ruby.framework/Versions//universal-darwin14/rbconfig.rb:: warning: Insecure world writable Re-creating CocoaPods due to major version update. Updating local specs repositories Analyzing dependencies [!] The dependency `ReactiveCocoa (= 2.1.8)` is not used in any concrete target. The dependency `objectiveflickr (= 2.0.4)` is not used in any concrete target. The dependency `LinqToObjectiveC (= 2.0.0)` is not used in any concrete target. The dependency `SDWebImage (= 3.6)` is not used in any concrete target.
3. 修改方法：
Podfile内容更改：
platform :ios, '7.0' target "RWTFlickrSearch" do pod 'ReactiveCocoa', '2.1.8' pod 'objectiveflickr', '2.0.4' pod 'LinqToObjectiveC', '2.0.0' pod 'SDWebImage', '3.6' end
这样再运行pod install，就会成功了。