macOS/BSD: fix Vagrantfile in-place editing
macOS/BSD: fix Vagrantfile in-place editing

* Unlike GNU-sed, BSD-sed `-i` option requires an explicit backup
  extension.
* Newlines are difficult to get right on different versions of `sed`.

This commit replaces the `sed` invocation with `perl`. It does the same
job with almost the same syntax, but avoids portability issues.

zamentur/yunohost-development,YunoHost/ynh-dev,YunoHost/ynh-dev