Fix db setup in installation doc

It was:

````markdown
3. Create databases, schemas and populate with seed data:
- bundle exec rake db:setup
-
- # - OR - in multiple steps:
# bundle exec rake db:create:all
# bundle exec rake db:migrate
# bundle exec rake db:seed
````

However, `bundle exec rake db:setup` results in the execution of the following commands:

* db:create
* db:schema:load
* db:seed

We don't commit the `schema.rb` file. Thus, `db:setup` won't work.