Remove 'services' directive from .travis.yml

According to the Travis docs at http://about.travis-ci.org/docs/user/database-setup/,
mysql and postgre are started on boot (and therefore don't need manual a start), while
we all know that sqlite is not a service and doesn't need that either.
diff --git a/.travis.yml b/.travis.yml
index 62acf05..a1635ea 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,11 +4,6 @@
   - 5.3
   - 5.4
   
-services:
-  - mysql
-  - postgresql
-  - sqlite
-
 env:
   - DB=mysql
   - DB=pgsql