Fix pip editable install on Travis CI (again...)
Created by: paulromano
Seems like the pip folks can't make up their minds. A new release came out yesterday that changed pip such that you don't need to specify
--no-use-pep517 when doing an editable install. So, I've reverted our travis install script to what it was before. I noticed this when I saw that that the tests on #1224 were failing.