Rails 5.1 Support

[felixbuenemann]

This adds Rails 5.1 to the CI matrix and fixes the following deprecation warnings with ActiveRecord 5.1:

DEPRECATION WARNING: The behavior of attribute_changed? inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after save returned (e.g. the opposite of what it returns now). To maintain the current behavior, use saved_change_to_attribute? instead. (called from update_parents_counter_cache at acts_as_tree/lib/acts_as_tree.rb:361)

DEPRECATION WARNING: The behavior of attribute_was inside of after callbacks will be changing in the next version of Rails. The new return value will reflect the behavior of calling the method after save returned (e.g. the opposite of what it returns now). To maintain the current behavior, use attribute_before_last_save instead. (called from update_parents_counter_cache at acts_as_tree/lib/acts_as_tree.rb:362)

Closes #67.