Set config file permissions to prevent group/world read

This ensures that when creating the authentication token file, the permission is set to owner-only read/write.

This doesn't change permissions on any existing files; I didn't see an obvious place to do that, and there are other implications to having the gem modify permissions that may have been changed intentionally, but I believe it's worth considering a warning that the permissions are lax, similar to the warning shown by `gem` for lax permissions on ~/.gem/credentials.