Improve test for nbconvert citation filter.

'import lxml' was succeeding, but 'from lxml import html' was failing.
Fix EvalFormatter test for Python 3.4

Changes to string formatting mean slicing in EvalFormatter now works.
Add comment about string formatting changes in Python 3.4
Merge pull request #4758 from takluyver/testing-py34

Python 3.4 fixes