travis: ignore SC2001 which doesn't work for $CHROOT_VARIABLES handling

SC2181 = See if you can use ${variable//search/replace} instead

This doesn't work for the $CHROOT_VARIABLES which includes
variables with strings like "/", so we can't rely on the
${variable//search/replace} approach. Quoting from
https://github.com/koalaman/shellcheck/wiki/SC2001:

| Utilizing some of the more complex capabilities of sed is
| required occasionally and it is safe to ignore SC2001.