Actually bump slapos.buildout in slapos "client-side" common file.

This was intended in 795cedcf, but it should not be needed to revert there.

stack/slapos.cfg

@@ -58,7 +58,7 @@ eggs =
 # https://github.com/SlapOS/hexagonit.recipe.download
 hexagonit.recipe.download = 1.5.1-dev-slapos-002
 # Use SlapOS patched zc.buildout
-zc.buildout = 1.6.0-dev-SlapOS-006
+zc.buildout = 1.6.0-dev-SlapOS-008
 
 [networkcache]
 download-cache-url = http://www.shacache.org/shacache