Fix download.sh to handle re-download

[dmitrizagidulin]

I saw that we had a new Pillar version and new Pharo 4.0 image.
I went to run ./download.sh. It downloaded the new image, but then errored out:

Downloading the latest pharoVM:
    http://files.pharo.org/vm/pharo/mac/stable.zip
mkdir: pharo-vm: File exists

I thought, hmm, ok, maybe I have to run ./clean.sh first.

$ ./clean.sh 
./clean.sh: line 19: ./pillar: No such file or directory

[dmitrizagidulin]

Also, why does download.sh seem to download sources for old Pharo versions?

Downloading the latest pharoVM:
    http://files.pharo.org/vm/pharo/mac/stable.zip
pharo-vm/Pharo.app/Contents/MacOS/Pharo
Downloading PharoV10.sources:
    http://files.pharo.org/sources//PharoV10.sources.zip
Downloading PharoV20.sources:
    http://files.pharo.org/sources//PharoV20.sources.zip
Downloading PharoV30.sources:
    http://files.pharo.org/sources//PharoV30.sources.zip
Downloading PharoV40.sources:
    http://files.pharo.org/sources//PharoV40.sources.zip

[jecisc]

This is something true for all the Pharo books. When we'll change that we have to change the others books too.