Kepp tornado pin for now

Author: Martin Durant

Dockerfile

@@ -19,7 +19,7 @@ ENV PATH="/work/bin:/work/miniconda/bin:$PATH"
 # Install pydata stack
 RUN conda config --set always_yes yes --set changeps1 no --set auto_update_conda no
 RUN conda install notebook=4.2 ipywidgets psutil numpy scipy pandas bokeh scikit-learn statsmodels pip numba \
-        scikit-image datashader holoviews nomkl matplotlib lz4 tornado
+        scikit-image datashader holoviews nomkl matplotlib lz4 tornado=4.4
 RUN conda install -c conda-forge fastparquet s3fs zict python-blosc cytoolz dask distributed jupyter_dashboards dask-searchcv \
  && conda clean -tipsy \
  && jupyter nbextension enable jupyter_dashboards --py --sys-prefix \