Add a -s option to dist.sh

This will allow one to repackage sagenb with only the changed
files in sagenb itself.
Otherwise, every time dist.sh is run, it redownloads all the
dependencies all over again. It is waste of time and bandwidth.

With this -s switch, you can avoid redownloading all the
dependencies while packaging sagenb.
Update the version to 0.10.8