Enable beta stability for project development only

Author: VolCh

composer.json

@@ -10,6 +10,7 @@
     "forum": "https://www.facebook.com/groups/phpwebdriver/",
     "source": "https://github.com/facebook/php-webdriver"
   },
+  "minimum-stability": "beta",
   "require": {
     "php": "^5.6 || ~7.0",
     "symfony/process": "^2.8 || ^3.1",