stack/slapos: use statsmodels 0.8.0 on python2

0.11.1 is not compatible with the version of numpy we are using on
python2:

  File "SR/develop-eggs/statsmodels-0.11.1-py2.7-linux-x86_64.egg/statsmodels/compat/numpy.py", line 46, in <module>
    NP_LT_114 = LooseVersion(np.__version__) < LooseVersion('1.14')
AttributeError: 'module' object has no attribute '__version__'