config/vm: cast box version to string before string operations

plugins/kernel_v2/config/vm.rb

@@ -581,7 +581,7 @@ module VagrantPlugins
           @hostname && @hostname !~ /^[a-z0-9][-.a-z0-9]*$/i
 
         if @box_version
-          @box_version.split(",").each do |v|
+          @box_version.to_s.split(",").each do |v|
             begin
               Gem::Requirement.new(v.strip)
             rescue Gem::Requirement::BadRequirementError

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

@@ -133,6 +133,14 @@ describe VagrantPlugins::Kernel_V2::VMConfig do
 
       assert_valid
     end
+
+    ["1", 1, "1.0", 1.0].each do |valid|
+      it "is valid: #{valid}" do
+        subject.box_version = valid
+        subject.finalize!
+        assert_valid
+      end
+    end
   end
 
   describe "#communicator" do