[local|storage] Fix Local::File deletion for Ruby 1.8.

Ruby 1.8 doesn't have a Dir.exists? method, which causes an exception and
prevents empty parent directories from being deleted. These changes use
File.exists? and File.directory? in place of Dir.exists?. Fixes
fog#707.

lib/fog/local/models/storage/file.rb

@@ -50,7 +50,7 @@ def destroy
               break
             end
             pwd = Dir.pwd
-            if ::Dir.exists?(dir_path)
+            if ::File.exists?(dir_path) && ::File.directory?(dir_path)
               Dir.chdir(dir_path)
               if Dir.glob('*').empty?
                 Dir.rmdir(dir_path)