knock lxml back to 3.2.3

dmt-celery builds have been failing, not because anything is actually
broken with lxml 3.4.0, but because compiling it requires more
memory than Krusty seems to have; so it keeps getting oom-killed
during the compile. :(
Merge pull request #177 from ccnmtl/lxml-downgrade

knock lxml back to 3.2.3

merging this one myself. just want to get dmt-celery builds working again before merging other stuff.