Switch Travis build to Xenial

Also explicitly start the mysql and pgsql services, as these are
no longer running by default.

Author: nikic

.travis.yml

@@ -1,4 +1,4 @@
-dist: trusty
+dist: xenial
 language: c
 sudo: required
 addons:
@@ -18,6 +18,11 @@ addons:
       - libxpm-dev
       - libzip-dev
 
+
+services:
+  - mysql
+  - postgresql
+
 notifications:
     email: 
        on_failure: change