Merge branch '5.0' into 5.1

* 5.0:
  [ErrorHandler] fix throwing from __toString()
  Removed comments and requirements relative to php <5.5 (not supported anymore)
  Fix caching of parent locales file in translator
  fix validating lazy properties that evaluate to null

Translator.php

@@ -397,7 +397,7 @@ private function loadFallbackCatalogues(string $locale): void
     protected function computeFallbackLocales(string $locale)
     {
         if (null === $this->parentLocales) {
-            $parentLocales = json_decode(file_get_contents(__DIR__.'/Resources/data/parents.json'), true);
+            $this->parentLocales = json_decode(file_get_contents(__DIR__.'/Resources/data/parents.json'), true);
         }
 
         $locales = [];
@@ -410,7 +410,7 @@ protected function computeFallbackLocales(string $locale)
         }
 
         while ($locale) {
-            $parent = $parentLocales[$locale] ?? null;
+            $parent = $this->parentLocales[$locale] ?? null;
 
             if ($parent) {
                 $locale = 'root' !== $parent ? $parent : null;