Use xenial and php 7.3

.travis.yml

@@ -1,18 +1,24 @@
 language: php
 
+dist: xenial
+
+services:
+  - mysql
+  - postgresql
+
 env:
   global:
     - COMPOSER_ROOT_VERSION=4.4.x-dev
 
 matrix:
   include:
-    - php: 5.6
-      env: DB=MYSQL
-    - php: 7.0
-      env: DB=PGSQL
     - php: 7.1
       env: DB=MYSQL
     - php: 7.2
+      env: DB=PGSQL
+    - php: 7.3
+      env: DB=MYSQL
+    - php: 7.3
       env: DB=MYSQL
 
 before_script: