fixes an exception issue when a non-directory appears in the boxes dir

lib/vagrant/box_collection.rb

@@ -133,7 +133,7 @@ module Vagrant
       results = []
 
       @logger.debug("Finding all boxes in: #{@directory}")
-      @directory.children(true).each do |child|
+      @directory.children(true).select(&:directory?).each do |child|
         box_name = child.basename.to_s
 
         # If this is a V1 box, we still return that name, but specify

test/unit/vagrant/box_collection_test.rb

@@ -135,6 +135,11 @@ describe Vagrant::BoxCollection do
       results = instance.all.sort
       results.should == [["bar", :virtualbox, :v1], ["foo", :vmware]]
     end
+
+    it 'does not raise an exception when a file appears in the boxes dir' do
+      Tempfile.new('a_file', environment.boxes_dir)
+      expect { instance.all }.to_not raise_error
+    end
   end
 
   describe "finding" do