minor updates to ignores

implemented in both `.gitignore` and `layout_test.rb`

.gitignore

@@ -2,5 +2,7 @@
 /.bundle/
 
 # anywhere in the tree
+*~
+.DS_Store
 .ruby-version
 coverage

test/layout_test.rb

@@ -5,7 +5,7 @@ describe "Repo layout" do
 
   # todo: a more clever way to do this would be to dispense with
   #       the imperfect IGNORE lists and read the actual repo
-  #       contents by running "git ls-files
+  #       contents by reading the output of "git ls-files"
 
   # dot dirs are always a project of Dir.entries
   # other files are items that the developer hopefully has gitignored