Update test php versions

Signed-off-by: Julius Härtl <jus@bitgrid.net>

.travis.yml

@@ -2,11 +2,15 @@ language: php
 services:
     - mysql
 php:
-    - 7.2
     - 7.3
 env:
     - CORE_BRANCH=master DB=mysql
 
+matrix:
+  include:
+  - php: 7.2
+    env: "DB=sqlite CORE_BRANCH=stable20"
+
 before_install:
     - export PATH="$PWD/vendor/bin:$PATH"
     - phpunit --version