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.

To support both old and new versions of the plugin, check if the
TimedProvision class is defined before using it.
Merge pull request #99 from tmatilai/aws-0.4

Compatibility with vagrant-aws v0.4.0