Mark box_collection_test to require bsdtar

These tests implicitly require bsdtar and should be skipped if the binary is not
available.

test/unit/vagrant/box_collection_test.rb

@@ -3,7 +3,7 @@ require File.expand_path("../../base", __FILE__)
 require "pathname"
 require 'tempfile'
 
-describe Vagrant::BoxCollection, :skip_windows do
+describe Vagrant::BoxCollection, :skip_windows, :bsdtar do
   include_context "unit"
 
   let(:box_class)   { Vagrant::Box }