Fix Rails 4 unknown key :condition in has_many relation definition

Hi, I'm using Rails 4.1.1 and it throws an exception:
```
Unknown key: :conditions. Valid keys are: :class_name, :class, :foreign_key, :validate, :autosave, :table_name, :before_add, :after_add, :before_remove, :after_remove, :extend, :primary_key, :dependent, :as, :through, :source, :source_type, :inverse_of, :counter_cache (ArgumentError)
```
Rails 4 has changed the way conditions to a scope or a relation, from :conditions to ->.
This quickfix solves the problem.
Maybe it'd be better to create another branch for rails 4+, because I don't know if this syntax is compatible with rails 3.