update Vagrantfile to deal with Vagrant ≥ 2.1.3

Vagrant changed how it deals with plugins in version 2.1.3; see hashicorp/vagrant#10161.

Unclear whether the "old" way of checking for plugins will return in 2.1.5 or later.

A workaround is to comment out the loop that check whether plugins are installed.

There is also a new syntax that more clearly describes our needs and allows for semi-automated installation; see OXID-eSales/oxvm_base#22.