Make sure the Doctrine\Persistence namespace exists

The new namespace with a BC layer was introduced in 1.3.5

Author: eigan

composer.json

@@ -25,7 +25,8 @@
   "require": {
     "php": ">=7",
     "doctrine/orm": "~2.6.0|~2.7.0",
-    "doctrine/inflector": "^1.1"
+    "doctrine/inflector": "^1.1",
+    "doctrine/persistence": "^1.3.5|^2.0"
   },
   "require-dev": {
     "phpunit/phpunit": "~5.0",