Update DEVSTACK_SERIES to yoga

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

Change-Id: I0c7809bdac6482bb947f394b0c2535fabb4cf067
diff --git a/stackrc b/stackrc
index 16aa93a..3dc800a 100755
--- a/stackrc
+++ b/stackrc
@@ -247,7 +247,7 @@
 # Setting the variable to 'ALL' will activate the download for all
 # libraries.
 
-DEVSTACK_SERIES="xena"
+DEVSTACK_SERIES="yoga"
 
 ##############
 #