CocoaPods报错：The dependency `` is not used in any concrete target

报错描述： 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。

解决方法：
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/2.0/usr/lib/ruby/2.0.0/universal-darwin14/rbconfig.rb:213: warning: Insecure world writable dir /Applications/Cocos/tools/ant/bin in PATH, mode 040777Re-creating CocoaPods due to major version update.Updating local specs repositoriesAnalyzing 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。