Update CocoaPods script to use latest version

README.md

@@ -145,7 +145,7 @@ There are three ways to use SDWebImage in your project:
 #### Podfile
 ```
 platform :ios, '7.0'
-pod 'SDWebImage', '~>3.8'
+pod 'SDWebImage', '~> 4.0'
 ```
 
 If you are using Swift, be sure to add `use_frameworks!` and set your target to iOS 8+: