Pin html5lib requirement to avoid install conflict

  Downloading html5lib-0.999999999.tar.gz (245kB)
        Complete output from command python setup.py egg_info:
            html5lib requires setuptools version 18.5 or above; please
            upgrade before installing (you have 18.2)

requirements.txt

@@ -3,3 +3,4 @@ messytables>=0.12.0
 python-slugify
 Requests
 argparse
+html5lib==0.9999999