Revert has_many due to ActiveResource version

It turns out the version of `ActiveResource` bundled with `Rails 3.2` is missing `Associations`.

<https://github.com/rails/rails/blob/3-2-stable/activeresource/lib/active_resource.rb> vs <https://github.com/rails/activeresource/blame/master/lib/active_resource/base.rb#L19>

This PR reverts the `has_many` and puts a method called `customers` which you can pass it params (if needed.)

### Review

@dylanahsmith @alexcoco