Merge branch '3.1'

* 3.1:
  [travis] Test on PHP 7.1
  simplified code
  Remove unused variable
  3.0 Upgrade Guide: Added Table of Contents
  [HttpFoundation] add missing upgrade entry
  [FrameworkBundle] Convert null prefix to an empty string in translation:update command
  Improved the deprecation messages for service aliases
  [Serializer] minor: Remove an extra space in a YamlFileLoader error
  [HttpKernel] Clean ArgumentMetadataFactory::getType()
  [PropertyInfo] Fix edge cases in ReflectionExtractor

Mapping/Loader/YamlFileLoader.php

@@ -75,7 +75,7 @@ public function loadClassMetadata(ClassMetadataInterface $classMetadata)
 
                     if (isset($data['max_depth'])) {
                         if (!is_int($data['max_depth'])) {
-                            throw new MappingException('The "max_depth" value must an integer  in "%s" for the attribute "%s" of the class "%s".', $this->file, $attribute, $classMetadata->getName());
+                            throw new MappingException('The "max_depth" value must be an integer in "%s" for the attribute "%s" of the class "%s".', $this->file, $attribute, $classMetadata->getName());
                         }
 
                         $attributeMetadata->setMaxDepth($data['max_depth']);