Trying to see what was wrong in the old script

Fixes #77 (closed)

This one seems to be working. See for example:

# .../bob.admin/conda/clean-betas.py .../conda/label/beta | grep bob.io.video
==== packages for bob.io.video/py27 (3) ====
remove .../conda/label/beta/linux-64/bob.io.video-2.0.20b0-py27h6550771_13.tar.bz2 (time=1520454985)
remove .../conda/label/beta/linux-64/bob.io.video-2.0.20b0-py27h6550771_14.tar.bz2 (time=1520865493)
[keep] .../conda/label/beta/linux-64/bob.io.video-2.0.20b0-py27h6550771_15.tar.bz2 (time=1521006061)
==== packages for bob.io.video/py36 (4) ====
remove .../conda/label/beta/linux-64/bob.io.video-2.0.20b0-py36h6550771_13.tar.bz2 (time=1520541353)
remove .../conda/label/beta/linux-64/bob.io.video-2.0.20b0-py36h6550771_14.tar.bz2 (time=1520800616)
remove .../conda/label/beta/linux-64/bob.io.video-2.0.20b0-py36h6550771_15.tar.bz2 (time=1520865493)
[keep] .../conda/label/beta/linux-64/bob.io.video-2.0.20b0-py36h6550771_16.tar.bz2 (time=1521006062)
==== packages for bob.io.video/py27 (3) ====
remove .../conda/label/beta/osx-64/bob.io.video-2.0.20b0-py27h5223d36_0.tar.bz2 (time=1520865491)
remove .../conda/label/beta/osx-64/bob.io.video-2.0.20b0-py27h5223d36_1.tar.bz2 (time=1520888688)
[keep] .../conda/label/beta/osx-64/bob.io.video-2.0.20b0-py27h5223d36_2.tar.bz2 (time=1521006060)
==== packages for bob.io.video/py36 (3) ====
remove .../conda/label/beta/osx-64/bob.io.video-2.0.20b0-py36h5223d36_14.tar.bz2 (time=1520369938)
remove .../conda/label/beta/osx-64/bob.io.video-2.0.20b0-py36h5223d36_15.tar.bz2 (time=1520865492)
[keep] .../conda/label/beta/osx-64/bob.io.video-2.0.20b0-py36h5223d36_16.tar.bz2 (time=1521006061)

merged

mentioned in commit 30b87cad