Compatibility with vagrant-aws v0.4.0

Next vagrant-aws release will remove its custom `TimedProvision` class and use only the standard `Builtin::Provision` instead: mitchellh/vagrant-aws@dd17f23.

To support both old and new versions of the plugin, check if the `TimedProvision` class is defined before using it.

This PR is based on v1.3.3 tag (in the hope of a point release :))  but it merges cleanly to the current master, too.