add mysql service for travis-ci

see http://about.travis-ci.org/blog/august-2012-upcoming-ci-environment-updates

.travis.yml

@@ -9,6 +9,8 @@ branches:
     - 'master'
 rvm:
   - 1.9.3
+services:
+  - mysql
 before_script:
   - "cp config/database.yml.$DB config/database.yml"
   - "cp config/gitlab.yml.example config/gitlab.yml"