Docker image building failure while building `pip install`

[koukyo1994]

Hello! Thank you for a great work with awesome datasets and nice implementation!!

I've found an issue while trying to build the Dockerfile.
It seems that something is wrong when install Python packages while conducting pip install -r requirements.txt.

Here's the error log of the failing position.

Successfully built backcall lmdb networkx pandocfilters prometheus-client pyrsistent toolz tornado visdom
mkl-random 1.0.1 has requirement intel-numpy<1.15,>=1.14, but you'll have intel-numpy 1.15.1 which is incompatible.
Installing collected packages: attrs, backcall, webencodings, bleach, certifi, cffi, cloudpickle, cycler, Cython, dask, decorator, defusedxml, entrypoints, idna, tornado, ipython-genutils, traitlets, python-dateutil, pyzmq, jupyter-core, jupyter-client, ptyprocess, pexpect, Pygments, pickleshare, wcwidth, prompt-toolkit, parso, jedi, ipython, ipykernel, MarkupSafe, Jinja2, joblib, pyrsistent, jsonschema, terminado, Send2Trash, nbformat, testpath, pandocfilters, mistune, nbconvert, prometheus-client, notebook, jupyterlab-server, jupyterlab, kiwisolver, lmdb, numpy, pyparsing, matplotlib, intel-openmp, icc-rt, tbb, tbb4py, mkl, mkl-random, intel-numpy, mkl-fft, networkx, pytz, pandas, Pillow, protobuf, pyOpenSSL, PyWavelets, requests, scipy, scikit-image, Shapely, svgwrite, svgpathtools, tensorboardX, toolz, tqdm, websocket-client, visdom
  Found existing installation: certifi 2019.3.9
Cannot uninstall 'certifi'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.
The command '/bin/sh -c pip install -r requirements.txt' returned a non-zero code: 1

I've dealt with the problem by changing the statement in requirements.txt from

certifi==2018.10.15

to

certifi==2019.3.9

but it may be better not to fix the version of certifi module.