Update DEVSTACK_SERIES to xena

stable/wallaby branch has been created now and
current master is for xena.

Change-Id: I42f67361fe50795d929752434342effddf123486
diff --git a/stackrc b/stackrc
index 648a028..9630221 100644
--- a/stackrc
+++ b/stackrc
@@ -245,7 +245,7 @@
 # Setting the variable to 'ALL' will activate the download for all
 # libraries.
 
-DEVSTACK_SERIES="wallaby"
+DEVSTACK_SERIES="xena"
 
 ##############
 #