Escape arguments to sed in scripts/debian-update-versions

Before, when running `rebuild`, it would abort with the following error:

    sed: -e expression #1, char 23: unknown option to `s'

in the script `scripts/debian-update-versions`. This happened because the sed expressions in the function `do_subst()` contained arguments that had slashes in them, which is a special character for sed. Escaping special sed characters solves this.