prevent stale snapshots on server

[philburk]

This issue is in response to #1139

The script that checks out the code is:

# Update repository
cd pa_repo
git pull
# update commit SHA in header file
./update_gitrevision.sh
cp src/common/pa_gitrevision.h ../.
git checkout -- src/common/pa_gitrevision.h
cd ../

Rather than doing a pull, we should delete and do a fresh clone.
Then, if git fails, the snapshot will fail and we will notice the error.

rm -rf pa_repo
git clone --depth 1   https://github.com/PortAudio/portaudio.git