Rails 5.1 compatibility, override default endpoints

Previously, the persistence and search modules used hard-coded values for RPC endpoints: :create, :delete, :destroy, :search, :update.

Add DSL methods so that these default values can be overridden:

class Developer < ActiveRemote::Base
  endpoint_for_create :register
  endpoint_for_destroy :delete
end

Also dial in Rails version since ActiveRemote is not yet compatible with Rails v5.2.