Revert "Update composer.json: Use Symfony YAML 3.1 and later"

Symfony YAML >= 3.0 requires PHP >= 5.5.9 and we're currently not planning to raise Pico's PHP requirement.

This reverts commit f251bc83ec995d05e0e33140df22a451c0cfe4af.

composer.json

@@ -26,7 +26,7 @@
         "php": ">=5.3.6",
         "twig/twig": "^1.18",
         "erusev/parsedown-extra": "^0.7",
-        "symfony/yaml" : "^3.1"
+        "symfony/yaml" : "^2.3"
     },
     "require-dev" : {
         "phpdocumentor/phpdocumentor": "^2.8",