Fix test that fails on 1.9 due to behaviour of String#start_with?

On ruby 1.9, the argument of String#start_with? does not automatically
get coerced into a string. In Homesick#clone, we use start_with? with a
Pathname instance, which now always returns false.

lib/homesick.rb

@@ -22,7 +22,7 @@ class Homesick < Thor
       destination = nil
       if File.exist?(uri)
         uri = Pathname.new(uri).expand_path
-        if uri.to_s.start_with?(repos_dir)
+        if uri.to_s.start_with?(repos_dir.to_s)
           raise "Castle already cloned to #{uri}"
         end