Add dev dependencies for building python lxml

When you try and `pip install lxml` it builds the source package. This
requires 3 dev packages be installed on the system.

See http://stackoverflow.com/a/6504860

See https://www.pivotaltracker.com/story/show/72070958

[#72070958]
Merge pull request #436 from alphagov/add-python-lxml-deps-to-backend

Add dev dependencies for building python lxml