Process logjam_database.yml with ERB

This makes using ENV vars possible in logjam_database.yml, so your deployment can conform to http://12factor.net/

My `logjam_database.yml` looks like this:

```yaml
default: &default
  host:     <%= ENV['LOGJAM_DB_HOST']     || 'localhost' %>
  password: <%= ENV['LOGJAM_DB_PASSWORD'] || '' %>
  port:     <%= ENV['LOGJAM_DB_PORT']     || '27017' %>
  username: <%= ENV['LOGJAM_DB_USERNAME'] || 'logjam' %>

development:
  default:
    <<: *default

test:
  default:
    <<: *default

production:
  default:
    <<: *default

staging:
  default:
    <<: *default

benchmarking:
  default:
    <<: *default
```