Remove beautifulsoup4 from dev_requirements.txt

[alycejenni]

It doesn't appear to be used anywhere and causes errors when used with Python 3.5+ (see ckan/ckanext-dcat#192).

(duplicate of #194 because I renamed the branch and accidentally closed the original PR, sorry :upside_down_face: )

[amercader]

Thanks @alycejenni