Listen file changes with the polling method

Some filesystems won't work without polling (VM/Vagrant Shared folders, NFS, Samba, sshfs, etc.).
https://github.com/guard/listen#issues--limitations

In this situation we introduce a new flag --polling to force the polling method on the Listen gem when using retest

Examples:

$ retest --polling
$ retest --polling --all 
$ retest --polling 'bundle exec rubocop <changed>'