doc: dynamically construct version

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
diff --git a/doc/conf.py b/doc/conf.py
index 138fd6d..dc2f9f0 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -58,9 +58,10 @@
 # built documents.
 #
 # The short X.Y version.
-version = u'v1.20'
+import subprocess
+version = subprocess.check_output("git describe", shell=True).decode()
 # The full version, including alpha/beta/rc tags.
-release = u'v1.20'
+release = version
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.