Added Rails 4 support

ActiveRecord 4 requires all scopes to have a callable object (lambda/proc). This change fixes the scope syntax for AR 3+ and prevents the following warning that the current gem generates when scopes are enabled:

```DEPRECATION WARNING: Calling #scope or #default_scope with a hash is deprecated. Please use a lambda containing a scope. E.g. scope :red, -> { where(color: 'red') }.```