upgrade(rails 5): no longer use rake to invoke rake tasks, use rails

though I left the case alone where the command would previously been
just `bundle exec rake`, as doing the same doesn't work for rails.

Rakefile

@@ -1,7 +1,6 @@
-#!/usr/bin/env rake
 # Add your own tasks in files placed in lib/tasks ending in .rake,
 # for example lib/tasks/capistrano.rake, and they will automatically be available to Rake.
 
-require File.expand_path('../config/application', __FILE__)
+require_relative 'config/application'
 
-Railsgoat::Application.load_tasks
+Rails.application.load_tasks