fix #379 README: using the version of guzzle6-adapter containing httplug v1.x

Author: gserrg

README.md

@@ -15,7 +15,7 @@ Installation
 Via [composer](https://getcomposer.org)
 
 ```bash
-composer require m4tthumphrey/php-gitlab-api php-http/guzzle6-adapter
+composer require m4tthumphrey/php-gitlab-api php-http/guzzle6-adapter:^1.0
 ```
 
 Why `php-http/guzzle6-adapter`? We are decoupled from any HTTP messaging client with help by [HTTPlug](http://httplug.io).