Remove panther from the pack

It brings facebook/web-driver, which brings ext-curl,ext-zip, etc

Author: nicolas-grekas

composer.json

@@ -7,7 +7,6 @@
         "php": "^7.0",
         "symfony/browser-kit": "*",
         "symfony/css-selector": "*",
-        "symfony/panther": "*",
         "symfony/phpunit-bridge": "*"
     }
 }