Add lxml >= 4.0 to setup.py install_requires
Add lxml >= 4.0 to setup.py install_requires

fixes https://travis-ci.org/robofab-developers/fontParts/jobs/399648345#L492

See discussion at https://github.com/unified-font-object/ufoLib/pull/153#issuecomment-402205299

googlefonts/fonttools,fonttools/fonttools