Raise if versions are unpicked (unpinned)

If the version is unpinned, the buildout will fail. It is currently mandatory to
  keep all the eggs pinned.

  Any development environment can se allow-picked-version to true if they were tryting to check
  upgrades of some eggs.

This were already discussed and agreeded and the test already passed:

[20180611-4AD45F1D](https://nexedijs.erp5.net/#/test_result_module/20180611-4AD45F1D)

/cc @jerome @jm 

/reviewed-on nexedi/slapos!363

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, fail if eggs are not pinned.
+allow-picked-versions = false
+
 # Use shacache and lxml
 extends =
   ../component/git/buildout.cfg