Fixed version number in README.md.

Author: jepf

README.md

@@ -14,7 +14,7 @@ Additionally, the API client needs PHP 5.6 or newer.
 ### Integration into your project
 Add the following to the "require" section of your project's composer.json file:
 ```json
-"zammad/zammad-api-client-php": "1.5.*"
+"zammad/zammad-api-client-php": "1.6.*"
 ```
 
 ### Installing the API client's dependencies