remove attr_accessible definitions

Error: `attr_accessible` is extracted out of Rails into a gem. Please
use new recommended protection model for params(strong_parameters) or
add `protected_attributes` to your Gemfile to use old one.
fix has_many association options deprecation warnings
change deprecated .all to .load
due to associations definitions change, force rails ~> 4
--amend
change includes for joins, as it loads the association
require active support core ext to have mattr_accessor defined
remove some scoped warnings