Allow failures in Travis with PHP 7.3

Because our dependency Gaufrette contains a PHP 7.3 incompatibility:

There was 1 error:
1) Burzum\FileStorage\Test\TestCase\Model\Behavior\FileStorageBehaviorTest::testAfterDelete
"continue" targeting switch is equivalent to "break". Did you mean to use "continue 2"?
/home/travis/build/burzum/cakephp-file-storage/vendor/knplabs/gaufrette/src/Gaufrette/Util/Path.php:30
/home/travis/build/burzum/cakephp-file-storage/vendor/knplabs/gaufrette/src/Gaufrette/Adapter/Local.php:37
/home/travis/build/burzum/cakephp-file-storage/src/Storage/StorageManager.php:171
/home/travis/build/burzum/cakephp-file-storage/src/Storage/StorageManager.php:148
/home/travis/build/burzum/cakephp-file-storage/src/Storage/StorageManager.php:126
/home/travis/build/burzum/cakephp-file-storage/src/Storage/StorageTrait.php:44
/home/travis/build/burzum/cakephp-file-storage/src/Model/Behavior/FileStorageBehavior.php:161
/home/travis/build/burzum/cakephp-file-storage/tests/TestCase/Model/Behavior/FileStorageBehaviorTest.php:107

.travis.yml

@@ -14,6 +14,7 @@ env:
 
 matrix:
   allow_failures:
+    - php: 7.3
     - php: nightly
 
   fast_finish: true