a.rb

--- a/core/lib/Drupal/Core/Http/Client.php
+++ b/core/lib/Drupal/Core/Http/Client.php
@@ -20,28 +20,27 @@ class Client extends GuzzleClient {
    * {@inheritdoc}
    */
   public function __construct(array $config = []) {
-    $defaults = array(
+    $default_config = array(
       'config' => array(
         'curl' => array(
           CURLOPT_TIMEOUT => 30,
           CURLOPT_MAXREDIRS => 3,
         ),
-        // Security consideration: we must not use the certificate authority file
-        // shipped with Guzzle because it can easily get outdated if a certificate
-        // authority is hacked. Instead, we rely on the certificate authority file
-        // provided by the operating system which is more likely going to be
-        // updated in a timely fashion.
-        'ssl' => array(
-          'certificate_authority' => 'system',
-        ),
       ),
       'defaults' => array(
+        // Security consideration: we must not use the certificate authority
+        // file shipped with Guzzle because it can easily get outdated if a
+        // certificate authority is hacked. Instead, we rely on the certificate
+        // authority file provided by the operating system which is more likely
+        // going to be updated in a timely fashion. This overrides the default
+        // path to the pem file bundled with Guzzle.
+        'verify' => TRUE,
         'headers' => array(
           'User-Agent' => 'Drupal (+http://drupal.org/)',
         ),
       ),
     );
-    $config = NestedArray::mergeDeep($defaults, $config);
+    $config = NestedArray::mergeDeep($default_config, $config);
 
     parent::__construct($config);