Don't add puppetlabs sources for lucid

They no longer exist

spec/acceptance/apt_spec.rb

@@ -23,8 +23,9 @@ describe 'apt class' do
           'preferences'    => true,
           'preferences.d'  => true,
         },
-        sources => {
-          'puppetlabs' => {
+        sources => ? $::lsbdiscodename {
+          'lucid' => undef,
+          default => { 'puppetlabs' => {
               'ensure'   => present,
               'location' => 'http://apt.puppetlabs.com',
               'repos'    => 'main',
@@ -34,6 +35,7 @@ describe 'apt class' do
               },
             },
           },
+        },
       }
       EOS