core: use keyword args to simplify BoxCollection#add

lib/vagrant/box_collection.rb

@@ -70,7 +70,7 @@ module Vagrant
     #   the box represents will be added.
     # @param [Boolean] force If true, any existing box with the same name
     #   and provider will be replaced.
-    def add(path, name, formats=nil, force=false)
+    def add(path, name, formats=nil, force: false)
       formats = [formats] if formats && !formats.is_a?(Array)
       provider = nil
 

test/unit/vagrant/box_collection_test.rb

@@ -72,7 +72,7 @@ describe Vagrant::BoxCollection do
 
       # Attempt to add the box with the same name
       box_path = environment.box2_file(prev_box_provider, metadata: { "replaced" => "yes" })
-      box = instance.add(box_path, prev_box_name, nil, true)
+      box = instance.add(box_path, prev_box_name, nil, force: true)
       box.metadata["replaced"].should == "yes"
     end