Add lxml to requirements.txt

BeautifulSoup4 has a soft dependency on lxml. If present, it will use
lxml.html, which has different behavior than the default parser. For
example, lxml.html ensures that <br> tags will never have any content,
while the default parser allows nesting text and tags inside <br>.
(see https://groups.google.com/d/msg/beautifulsoup/XHjDzPseD9U/3CEF7jBT_s8J)