Update pandas to 0.21.0

[yet-another-account]

Without this, the following issue is thrown:

RuntimeError: module compiled against API version 0xb but this version of numpy is 0xa
Traceback (most recent call last):
  File "extracter_script.py", line 1, in <module>
    from helpers.extracter import Spider
  File "/home/eukaryote/workspace/deep-summarization/helpers/extracter.py", line 2, in <module>
    import pandas as pd
  File "/home/eukaryote/workspace/deep-summarization/local/lib/python2.7/site-packages/pandas/__init__.py", line 31, in <module>
    "extensions first.".format(module))
ImportError: C extension: umpy.core.multiarray failed to import not built. If you want to import pandas from the source directory, you may need to run 'python setup.py build_ext --inplace' to build the C extensions first.