Upgrade lxml

This brings CCHQ up to date on lxml, as we've had some problems in the tests that cannot be attributed to our code and are likely already-fixed lxml bugs.

Note that CCHQ's test coverage is low enough that I do not have perfect confidence that this does not introduce any issues. However both casexml and couchforms depend on "lxml" with no version so their Travis-CI builds have been tested against the latest lxml for some time.