Add ability to set a due date on a Task

* created migration file to add due_at column to tasks table
* made due_at attribute accessible on Task model
* added a field to enter due date on task form
* added due date column to HTML table in List Tasks page
Add Hirb to Gemfile
edited migration file to add due_at column *after the name column* in tasks table
Add ability to sort Tasks by due date

Code based on Railscast 228. I hope this is OK. I had done this before
and thought it’s a simple, perfect solution. There was no need to watch
the Railscast, I just had to look at the code.
Update the ListsController so that every action responds to JSON
Skip locale validation, silence I18n.enforce_available_locales deprecation warning when running spec
added simple rspec tests for ListsController: index, show and edit actions should respond to JSON
added rspec test for Task model due_at attribute should be accessible