Makefile: use setup.py for getting the current versions

Under some circumstances, getting the version indirectly through
`avocado/core/version.py` can return `unknown.unknown`.  Let's use the
authoritative source for version numbers: `setup.py` itself.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

Makefile

@@ -12,7 +12,7 @@
 
 PYTHON=$(shell which python)
 PYTHON26=$(shell $(PYTHON) -V 2>&1 | grep 2.6 -q && echo true || echo false)
-VERSION=$(shell $(PYTHON) $(CURDIR)/avocado/core/version.py)
+VERSION=$(shell $(PYTHON) setup.py --version)
 DESTDIR=/
 BUILDIR=$(CURDIR)/debian/avocado
 PROJECT=avocado