(packaging) Use the packaging loader for tasks

The packaging repo now uses an explicit loader which handles loading the
various rake tasks in packaging. This commit updates the puppet-dashboard
Rakefile to use the loader instead of a blind glob of the ext/packaging/tasks
directory. We move this load into the rescue LoadError block because the
packaging repo won't always be there, and define RAKE_ROOT and use it since
its used in more than one place in the Rakefile.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
(packaging) Remove Ubuntu Oneiric from build targets

Oneiric is EOL, and we are no longer building for it. This commit removes the
Oneiric target from the build list for puppet-dashboard.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>