[5.7] Add PHP 7.3 to Travis (#26328)

* add php 7.3 to travis

* Update .travis.yml

* Require Guzzle 6.3

Author: andreybolonin

.travis.yml

@@ -13,6 +13,9 @@ matrix:
     - php: 7.2
     - php: 7.2
       env: setup=lowest
+    - php: 7.3
+    - php: 7.3
+      env: setup=lowest
 
 sudo: false
 

composer.json

@@ -77,6 +77,7 @@
         "aws/aws-sdk-php": "^3.0",
         "doctrine/dbal": "^2.6",
         "filp/whoops": "^2.1.4",
+        "guzzlehttp/guzzle": "^6.3",
         "league/flysystem-cached-adapter": "^1.0",
         "mockery/mockery": "^1.0",
         "moontoast/math": "^1.1",