add 'class Chef' outer scope

clearly we need a rubocop rule for this.

this fixes this exception which we were throwing incorrectly:

[2014-10-28T02:57:54+00:00] FATAL: NameError: uninitialized constant
Chef::Application::Exceptions

using 'class Chef::Application' saves a tiny amount of typing and
indentation but does not open up the outer lexical scope and breaks
otherwise working code like this.