This is required so that buildout does not fallback to installing
non-dev egg if version of wendelin.core from -dev differs from what has
been pinned.

For example the following

    [buildout]
    extends =
      https://lab.node.vifib.com/nexedi/slapos/raw/1.0.12/software/wendelin/software-dev.cfg

    # pin wendelin.core-dev to latest assumed-good revision with ZBlk1 support
    [wendelin.core-repository]
    revision = c507d9009f59fec2041bac9c31c5b08a48d3897d

will install wendelin.core-0.4.egg from pypi instead of installing
c507d9009f59fec2041bac9c31c5b08a48d3897d from repository, because that
latter revision says it is already version 0.5 and 1.0.12 wendelin SR
pins wendelin.core to 0.4 .

So unpin wendelin.core from versions and let software-dev.cfg work
always.

/cc @klaus
/reviewed-by @Tyagov  (on nexedi/slapos!36)