fix: pod version (marcuswestin#297)

because 	`WVJBIframe.src = 'https://__bridge_loaded__';` is support 6.0 version, doesn't support 5.0 version.

Author: hhgz9527

README.md

@@ -37,7 +37,7 @@ Installation (iOS & OSX)
 Add this to your [podfile](https://guides.cocoapods.org/using/getting-started.html) and run `pod install` to install:
 
 ```ruby
-pod 'WebViewJavascriptBridge', '~> 6.0'
+pod 'WebViewJavascriptBridge'
 ```
 
 ### Manual installation