config: use travis_logs_[env] as the default database name

This prevents conflict with travis_[env], which is used by travis-core
et al.
db: provide default values for host and port when connecting
db: add migrations

Close #11.
Update .travis.yml to run migrations
Update config/travis.example.yml
migrations: add log removal data

This pulls in the changes from #14.
db: fix add_log_remove_data migration
Add db:status Rake task which prints migration status
Remove leftover `require` used in debugging
Gemfile: move 'rake' from development to default group

Rake is now needed in production in order to run migrations, so not just
a development dependency anymore.
database: use [] instead of fetch on config

Hashr implements #fetch, but it's the implementation inherited from
Hash, so it doesn't really work and always returns the default value.
database: the connection string should use 'user' instead of 'username'
Merge pull request #15 from travis-ci/hh-migrations

Add migrations