pass default_scope a lambda for Rails 4 compability

In Rails 4 all ActiveRecord scopes must be defined with a callable object (i.e. Proc or lambda).  This commit will remove the warning.

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') }. (called from has_soft_deletion at /usr/local/rvm/gems/ruby-2.0.0-p247@global/bundler/gems/soft_deletion-bcba8464d3c4/lib/soft_deletion/setup.rb:27)