Update triggers usage for Vagrant 2.1.0

[becw]

• Requires using Vagrant >= 2.1.0
• Updates the default Vagrantfile to use the triggers functionality from Vagrant core instead of vagrant-triggers

To update Vagrant:

warning: this means that you won't be able to use Vagrantfiles that require vagrant-triggers

1. Uninstall the vagrant-triggers plugin with vagrant plugin uninstall vagrant-triggers
2. Update you Vagrant to >= 2.1.0, either by downloading from vagrantup.com, or if you use homebrew cask, brew cask reinstall vagrant
3. Repair/reinstall your other Vagrant plugins with vagrant plugin repair (this is generally required after updating Vagrant)

To update a project with the changes from this PR:

1. Get this code in the project (composer require whatever)
2. Reinstall the-vagrant: vendor/bin/the-vagrant-installer
3. Answer the prompts to regenerate your Vagrantfile
4. Examine the diff and make sure other customizations aren't removed from the file

This will not require destroying the vagrant box for your project.

Concerns

The problem with this update is that any projects using it will require that developers have Vagrant >= 2.1.0, and many older projects will be requiring Vagrant before 2.1.0 -- so we'll probably need to update those other projects as close to in sync as possible with when developers update their Vagrant installs.

I've added info to the wiki on updating to Vagrant >= 2.1.0, but I'm still mulling over how to handle this team-wide update.