core: Allow direct box adding again

lib/vagrant/action/builtin/handle_box.rb

@@ -66,10 +66,13 @@ module Vagrant
           box_formats = machine.provider_options[:box_format] ||
             machine.provider_name
 
+          version_ui = machine.config.vm.box_version
+          version_ui ||= ">= 0"
+
           env[:ui].output(I18n.t(
             "vagrant.box_auto_adding", name: machine.config.vm.box))
           env[:ui].detail("Box Provider: #{Array(box_formats).join(", ")}")
-          env[:ui].detail("Box Version: #{machine.config.vm.box_version}")
+          env[:ui].detail("Box Version: #{version_ui}")
 
           begin
             env[:action_runner].run(Vagrant::Action.action_box_add, env.merge({

plugins/kernel_v2/config/vm.rb

@@ -313,7 +313,7 @@ module VagrantPlugins
         @box_download_client_cert = nil if @box_download_client_cert == UNSET_VALUE
         @box_download_insecure = false if @box_download_insecure == UNSET_VALUE
         @box_url = nil if @box_url == UNSET_VALUE
-        @box_version = ">= 0" if @box_version == UNSET_VALUE
+        @box_version = nil if @box_version == UNSET_VALUE
         @graceful_halt_timeout = 60 if @graceful_halt_timeout == UNSET_VALUE
         @guest = nil if @guest == UNSET_VALUE
         @hostname = nil if @hostname == UNSET_VALUE

test/unit/plugins/kernel_v2/config/vm_test.rb

@@ -77,10 +77,10 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
   end
 
   context "#box_version" do
-    it "defaults to >= 0" do
+    it "defaults to nil" do
       subject.finalize!
 
-      expect(subject.box_version).to eq(">= 0")
+      expect(subject.box_version).to be_nil
     end
 
     it "errors if invalid version" do