Adding PDO as hard dependency as per discussion in #808

Ocramius · Ocramius · commit 36df6828b3b7 · 2017-05-03T14:01:41.000+02:00
diff --git a/composer.json b/composer.json
@@ -13,6 +13,7 @@
     ],
     "require": {
         "php": "^7.0",
+        "ext-pdo": "*",
         "doctrine/common": "^2.7.1"
     },
     "require-dev": {
