When installing from a git archive downloaded from github, the file
scapy/VERSION is not present since it is generated when building a
source archive with:

    ./setup.py sdist

To work around the problem, use the export-subst git attribute to make
git archive write the current revison information in scapy/__init__.py.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>