Simplify S3 helper.

Use S3 library to fetch the content length.
Remove unused memoization code.
Add log line when archiving log was successful.
Ack log message only if processed successfully.

Otherwise messages are dropped on the floor when they can't be
processed.
Retry processing log on connection pool timeout.
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