Carthage example uses v0.7 instead of master

Author: camelpunch

README.md

@@ -32,10 +32,10 @@ Test-driven from Swift and implemented in Objective-C, to avoid burdening Object
 1. Create a Cartfile with the following line:
 
    ```
-   github "rabbitmq/rabbitmq-objc-client" "master"
+   github "rabbitmq/rabbitmq-objc-client" "v0.7"
    ```
 
-   Replace `"master"` with the release, commit or branch of your choice.
+   Replace `"v0.7"` with the release, commit or branch of your choice.
 
    Run carthage, for example in a new project: