Update Travis config to include PHP 7.4 build, upgrade to Xenial

.travis.yml

@@ -1,6 +1,11 @@
 language: php
 
-dist: trusty
+dist: xenial
+
+services:
+  - mysql
+  - postgresql
+  - xvfb
 
 cache:
   directories:
@@ -20,6 +25,8 @@ matrix:
       env: DB=MYSQL PHPUNIT_TEST=1
     - php: 7.3
       env: DB=MYSQL PDO=1 PHPUNIT_COVERAGE_TEST=1
+    - php: 7.4
+      env: DB=MYSQL PHPUNIT_TEST=1
 
 before_script:
 # Init PHP