Create DB prior to compile assets (production instructions)

While the instructions correctly show to create the db for RAILS_ENV=development, they fail to repeat the procedure prior to `rake assets:precompile`, just after RAILS_ENV=production.

If this step isn't taken, nasty errors such as the following occur:

```
admin@discuss:~/discourse$ rake assets:precompile
/usr/bin/ruby1.9.1 /usr/local/bin/rake assets:precompile:all RAILS_ENV=production RAILS_GROUPS=assets
rake aborted!
FATAL:  database "discourse_production" does not exist
/var/lib/gems/1.9.1/gems/activerecord-3.2.12/lib/active_record/connection_adapters/postgresql_adapter.rb:1208:in `initialize'
```