MediaWiki-Vagrant requires at least vagrant 1.4.0

I didn't test with that exact version, but before 1.4.0,
the Vagrantfile would be loaded by vagrant plugin commands.

That causes problems, since setup.rb runs vagrant plugin list.

Also, explain how to check vagrant version, and minor wording tweaks.

Change-Id: I94a2ee78b3dd8d8abb61662aa4e1cd2c7cb44fc2