Gracefully handle unreadable directories

[owst]

If there is an unreadable directory considered by ripper-tags, an exception is raised:

.../lib/ripper-tags/data_reader.rb:59:in `open': Permission denied @ dir_initialize - foo/bar (Errno::EACCES)
	from .../lib/ripper-tags/data_reader.rb:59:in `entries'
	from .../lib/ripper-tags/data_reader.rb:59:in `resolve_file'
	from .../lib/ripper-tags/data_reader.rb:63:in `block in resolve_file'

(tested with 0.6.0)

This PR gracefully handles such directories, by skipping them (and logging if --verbose is given).

To reproduce the existing behaviour, run:

$ mkdir -p a/b
$ touch a/b/c.rb
$ chmod 355 a/b
$ ripper-tags --tag-file=/dev/null --recursive --verbose a # Errors

with this PR, we now have

$ bundle exec bin/ripper-tags --tag-file=/dev/null --recursive --verbose a
Ignoring unreadable directory `a/b'

[mislav]

Nice catch! Thank you for handling this.

[mislav]

Instead of checking File.readable? could we handle an exception from Dir.entries?

[owst]

Thanks for the review - sure, I've pushed a fixup to do that