Fix Travis tests

- Services are no longer started by default:
	https://docs.travis-ci.com/user/database-setup/

- Test on supported PHP versions:
	https://www.php.net/supported-versions.php

.travis.yml

@@ -1,9 +1,10 @@
+services:
+    - mysql
 language: php
 php:
-    - 7.0
-    - 5.6
-    - 5.5
-    - 5.4
+    - 7.1
+    - 7.2
+    - 7.3
 env:
     - DB=mysql
     - DB=pgsql