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