Restrict git describe to tags matching v*

Signed-off-by: Michael Marchetti <mmarchetti@sandvine.com>
diff --git a/setup.py b/setup.py
index 4fa3273..97d7b4b 100755
--- a/setup.py
+++ b/setup.py
@@ -41,7 +41,7 @@
 ]
 
 setup(name=_name,
-      version_command=('git describe', 'pep440-git'),
+      version_command=('git describe --match v*', 'pep440-git'),
       description = _description,
       long_description = open('README.rst').read(),
       author = _author,