Remove vagrant-triggers plugin

[stevenrombauts]

Since Vagrant 2.1 support for triggers has been added to the core Vagrant implementation. See: https://www.vagrantup.com/docs/triggers/

If you run the box now on 2.1.x, you'll get this warning:

WARNING: Vagrant has detected the `vagrant-triggers` plugin. This plugin conflicts
with the internal triggers implementation. Please uninstall the `vagrant-triggers`
plugin and run the command again if you wish to use the core trigger feature. To
uninstall the plugin, run the command shown below:

  vagrant plugin uninstall vagrant-triggers

Note that the community plugin `vagrant-triggers` and the core trigger feature
in Vagrant do not have compatible syntax.

To disable this warning, set the environment variable `VAGRANT_USE_VAGRANT_TRIGGERS`.

The built-in triggers mechanism is unfortunately not suited for our backup and restore workflow. Because it's absolutely vital that we ask users if they want to create a backup yes or no, we need to drop this feature entirely. Running inline ruby code is not possible with this new triggers method.

Todo's:

• Remove the triggers and the plugin requirement
• Update the FAQ to explain how to manually backup and restore.