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.

For example, on Nyarp we had a problem with database cleaner's delete strategy behaving differently locally and on Travis.

[1]: https://devcenter.heroku.com/articles/heroku-postgresql#version-support
[2]: http://docs.travis-ci.com/user/using-postgresql/