Suggest latest version of the mock client

The mock client stable first stable version has been released, and should be used because otherwise tests would be run with old versions of the php-http libraries.

Author: greg0ire

httplug/library-developers.rst

@@ -53,7 +53,7 @@ in the ``require-dev`` section in your library’s ``composer.json``. You could
  "php-http/client-implementation": "^1.0"
  },
  "require-dev": {
- "php-http/mock-client": "^0.3"
+ "php-http/mock-client": "^1.0"
  }
  }