Require at least symfony/css-selector 2.0.5 when testing with lowest deps

This helps as the autoloader in syfmony/css-selector `composer.json`
is configured only at 2.0.5.

Author: hkdobrev

composer.json

@@ -15,7 +15,8 @@
         "tijsverkoyen/css-to-inline-styles": "^1.5"
     },
     "require-dev" : {
-        "phpunit/phpunit": "^4.0|^5.0"
+        "phpunit/phpunit": "^4.0|^5.0",
+        "symfony/css-selector": "^2.0.5|^3.0"
     },
     "autoload": {
         "psr-4": {