Fix delegate. Update for ruby 2.6+ version

Author: vsevolod

.travis.yml

@@ -1,5 +1,6 @@
 services: mongodb
 language: ruby
+before_install: gem install bundler -v='1.11.2'
 script: "bundle exec rake spec:mongoid_6"
 rvm:
   - 2.3.0

lib/mongoid-observers.rb

@@ -10,6 +10,5 @@
 module Mongoid
   include ActiveModel::Observing
 
-  delegate(*ActiveModel::Observing::ClassMethods.public_instance_methods(false) <<
-    { to: Config })
-end
+  delegate ActiveModel::Observing::ClassMethods.public_instance_methods(false) => Config
+end