Merge branch '3.4' into 4.0

* 3.4:
  Bump ext-mongodb to 1.5 on Travis
  Redesign the Debug error page in prod
  [DI] fix dumping deprecated service in yaml
  bumped Symfony version to 3.4.13
  updated VERSION for 3.4.12
  updated CHANGELOG for 3.4.12
  bumped Symfony version to 2.8.43
  updated VERSION for 2.8.42
  update CONTRIBUTORS for 2.8.42
  updated CHANGELOG for 2.8.42

Author: nicolas-grekas

Dumper/YamlDumper.php

@@ -94,7 +94,7 @@ private function addService(string $id, Definition $definition): string
         }
 
         if ($definition->isDeprecated()) {
-            $code .= sprintf("        deprecated: %s\n", $definition->getDeprecationMessage('%service_id%'));
+            $code .= sprintf("        deprecated: %s\n", $this->dumper->dump($definition->getDeprecationMessage('%service_id%')));
         }
 
         if ($definition->isAutowired()) {

Tests/Fixtures/config/prototype.expected.yml

@@ -10,7 +10,7 @@ services:
         tags:
             - { name: foo }
             - { name: baz }
-        deprecated: %service_id%
+        deprecated: '%service_id%'
         arguments: [1]
         factory: f
     Symfony\Component\DependencyInjection\Tests\Fixtures\Prototype\Sub\Bar:
@@ -19,7 +19,7 @@ services:
         tags:
             - { name: foo }
             - { name: baz }
-        deprecated: %service_id%
+        deprecated: '%service_id%'
         lazy: true
         arguments: [1]
         factory: f