Shut up irritating `lxml` change warning.

Every time readability is run, it emits:  
>  /usr/local/lib/python3.4/dist-packages/readability_lxml-0.4.0.4-py3.4.egg/readability/htmls.py:110: FutureWarning: The behavior of this method will change in future versions. Use specific 'len(elem)' or 'elem is not None' test instead.

This is cause by checking the truth value of a lxml `Element`, which they're apparently trying to change. See http://stackoverflow.com/questions/20129996/why-does-boolxml-etree-elementtree-element-evaluate-to-false

Anyways, this replaces the truth-value check with a explicit check whether the node has any children.