upgrade documentation for new branch 2.1

Author: llaville

docs/getting-started.md

@@ -13,7 +13,7 @@ Install the Docker PHP toolbox with [Composer](https://getcomposer.org/).
 If you don't know yet what is composer, have a look [on introduction](http://getcomposer.org/doc/00-intro.md).
 
 ```shell
-composer require bartlett/docker-php-toolbox ^2.0
+composer require bartlett/docker-php-toolbox ^2.1
 ```
 
 ### With Git
@@ -22,5 +22,5 @@ The Docker PHP toolbox can be directly used from [GitHub](https://github.com/lla
 by cloning the repository into a directory of your choice.
 
 ```shell
-git clone -b 2.0 https://github.com/llaville/docker-php-toolbox.git
+git clone -b 2.1 https://github.com/llaville/docker-php-toolbox.git
 ```