stack/slapos: version ↑ slapos.buildout 2.5.2+slapos013

So that we can use
slapos.core!51

/reviewed-on !438

stack/slapos.cfg

@@ -100,7 +100,7 @@ eggs =
 [versions]
 setuptools = 33.1.1
 # Use SlapOS patched zc.buildout
-zc.buildout = 2.5.2+slapos012
+zc.buildout = 2.5.2+slapos013
 # Use SlapOS patched zc.recipe.egg (zc.recipe.egg 2.x is for Buildout 2)
 zc.recipe.egg = 2.0.3+slapos003
 # Use own version of h.r.download to be able to open .xz and .lz archives