Don't choke on unreadable directories

[andriokha]

I have an unreadable directory in my project root, and when trying to add a new dependency an error causes the change to be rolled back consistently.

Installation failed, reverting ./composer.json to its original content.

[Symfony\Component\Finder\Exception\AccessDeniedException]                                                                                          
  RecursiveDirectoryIterator::__construct(/path/to/project/unreadable): failed to open dir: Permission denied

[andriokha]

See #5

[jmolivas]

@andriokha PR #5 merged