Update AdExchange Buyer API examples to version v1.2.

Fixes issue #248.
diff --git a/samples/adexchangebuyer/sample_utils.py b/samples/adexchangebuyer/sample_utils.py
index 89f95ff..7083efa 100644
--- a/samples/adexchangebuyer/sample_utils.py
+++ b/samples/adexchangebuyer/sample_utils.py
@@ -108,5 +108,5 @@
   http = credentials.authorize(http)
 
   # Construct a service object via the discovery service.
-  service = build('adexchangebuyer', 'v1', http=http)
+  service = build('adexchangebuyer', 'v1.2', http=http)
   return service