Because of the concept introduced in here,
Rails::Plugin is nothing more than a Rails::Engine, but since it\'s loaded too late in the boot proces
i've just had the same problem here, just moved all the vendor/plugin/*/tasks/*.rake files to lib/tasks (had to create this directory because it didn't existed)
then rake rails:update ran fine
If you're the developer of a plugin, you would simply move
my_cool_plugin/tasks
to
my_cool_plugin/lib/tasks