Created by: wangkuiyi

Inspired by @luotao1 's https://github.com/PaddlePaddle/Paddle/pull/3419/files, which configures Travis-CI to install Python packages listed in a requirements.txt file, I think we should also make our Dockerfile install the enlisted packages.

Currently, our Dockerfile installs hard-written Python packages like .travis.yaml file : https://github.com/PaddlePaddle/Paddle/blob/develop/Dockerfile#L60

RUN pip install --upgrade pip && \
    pip install -U 'protobuf==3.1.0' && \
    pip install -U wheel pillow BeautifulSoup && \
    pip install -U docopt PyYAML sphinx && \
    pip install -U sphinx-rtd-theme==0.1.9 recommonmark && \
    pip install pre-commit 'requests==2.9.2' 'ipython==5.3.0' && \
    pip install 'ipykernel==4.6.0' 'jupyter==1.0.0' && \
    pip install opencv-python rarfile 'scipy>=0.19.0' 'nltk>=3.2.2'