Use PostgreSQL 9.3 on Travis CI

Heroku's default is Postgres 9.3 [1], but Travis' default is Postgres 9.1
[2]. This can occasionally cause problems which are difficult to debug.

[1]: https://devcenter.heroku.com/articles/heroku-postgresql#version-support
[2]: http://docs.travis-ci.com/user/using-postgresql/