Deal with vagrant-aws custom provision actions

vagrant-aws uses VagrantPlugins::AWS::Action::TimedProvision, which means
our provision tasks don't run on 'vagrant up'.

This is a dumb way to fix it, but it will work until we have something better.
Check if VagrantPlugin::AWS is defined
Merge pull request #31 from RiotGames/fix-vagrant-aws

Deal with vagrant-aws custom provision actions