Use Gmail-specific batch endpoint (#16)

* Specify batch path

* Add .travis.yml

.travis.yml

@@ -10,3 +10,4 @@ matrix:
 before_script:
   - composer require "swiftmailer/swiftmailer:${SWIFT_VERSION}" --no-update
   - composer install --no-interaction --dev
+

Services/Email.php

@@ -99,7 +99,7 @@ public function getBatch(string $userId, array $emailIds, array $options = []):
 
         $batchResponses = [];
         foreach (array_chunk($emailIds, 45)  as $gmailIds) {
-            $batchRequest = new \Google_Http_Batch($gmailBatchClient);
+            $batchRequest = new \Google_Http_Batch($gmailBatchClient, false, null, 'batch/gmail/v1');
             foreach ($gmailIds as $gmailId) {
                 /** @var RequestInterface $emailRequest */
                 $emailRequest = $gmailBatchService->users_messages->get($userId, $gmailId, $options);