[local|storage] Fix Local::File deletion for Ruby 1.8.
[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/fog#707.

fog/fog-local