[CI] pin datalad version
[CI] pin datalad version

Datalad version 0.15.0 and later requires a git-annex version that is not in the ubuntu default packages yet.
See also https://github.com/datalad/datalad/commit/315911ca76687ef0b8a529f0dfa99bf25fe040ba

samuelgarcia/python-neo,JuliaSprenger/python-neo,INM-6/python-neo,apdavison/python-neo,NeuralEnsemble/python-neo