tap: taps don't need to be Git repositories.

For example e.g. Boxen uses just a managed directory and puts formulae
into it.

Library/Homebrew/tap.rb

@@ -42,7 +42,7 @@ class Tap
   end
 
   def installed?
-    (@path/".git").directory?
+    @path.directory?
   end
 
   def custom_remote?