Simplify calculation scope building. Remove :order from associations as it is troublesome w/ calculation methods using postgresql.

Remove construct_calculation_arel_with_included_associations because it's same as construct_finder_arel_with_included_associations

Forgot to change named_scope to scope in an ArgumentError raise exception for duplication scopes related to commit d60bb0a9 [#3736 status:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Generators load path now will be Ruby load path. If you want to use rspec:install generator, you need generators/rspec/install_generator in your load path.