patch rails 4 schema.rb generation to include sharded tables

When rails generates db/schema.rb, it previously did not include any sharded tables. This adds an additional `ActiveRecord::Schema.define` block populated with the sharded tables. The sharded database will receive its own `schema_migrations` table when running `rake db:schema:load`.

This behavior currently only occurs on Rails 4+ / Ruby 2+

maybe keep it aligned inside the code by indenting and adding
```
<<HEADER.gsub(/^        /, "")
```