stack/slapos: disallow allow-picked-versions

from http://www.buildout.org/en/latest/reference.html :

> allow-picked-versions, default: ‘true’
> Indicate whether it should be possible to install requirements whose
> versions aren’t pinned

Setting this to false to make buildout refusing building SR where some
eggs would not be pinned.

stack/slapos.cfg

@@ -15,6 +15,10 @@ extensions +=
 
 show-picked-versions = true
 
+# To increase the chances of being able to rebuild profiles in the future,
+# by default, do not `allow-picked-versions` to fail if eggs are not pinned.
+allow-picked-versions = false
+
 # Use shacache and lxml
 extends =
   ../component/git/buildout.cfg