maven.yaml lint checking

Adjust indentation for credentials-binding section to match an example
in the jjb docs.  Not indented enough and value of credentials::file
manifested as null.  Local lint checking progressed farther, lets see
if this helps formal job generation.

Change-Id: Ia48cb3911435e38bdc884cc352815c88e8c69e32
diff --git a/Makefile b/Makefile
index 3478b0f..b205054 100644
--- a/Makefile
+++ b/Makefile
@@ -34,6 +34,7 @@
 include $(MAKEDIR)/include.mk
 
 VENV_DIR      ?= venv-jjb
+JJB_VERSION   ?= 2.8.0
 # JJB_VERSION   ?= 4.1.0
 JOBCONFIG_DIR ?= job-configs