Stub migrator so it doesn't fail in backwards compatibility tests

Author: sethboyles

spec/unit/lib/cloud_controller/db_migrator_spec.rb

@@ -78,6 +78,10 @@
   end
 
   describe 'benchmark' do
+    before do
+      allow(Sequel::Migrator).to receive(:run).with(any_args)
+    end
+
     let(:migrator) { DBMigrator.new(db) }
 
     it 'runs migrations without benchmark' do