Drop jessie build, add buster build

I could be wrong, but I don't think that LSCSoft supports jessie
anymore. However, buster is the current stable Debian release, so
it's useful to add a build for that distribution.
10 jobs for jessie-buster in 5 minutes and 5 seconds (queued for 24 seconds)
latest
Status Job ID Name Coverage
  Build
passed build:debian:buster #570393

00:02:02

passed build:debian:stretch #570392

00:02:04

passed build:el7 #570391

00:01:16

passed build:tarball #570390

00:00:27

 
  Test
passed docs #570398

00:01:14

passed syntax #570399

00:00:53

passed test:python2.7 #570394

00:00:44

passed test:python3.5 #570395

00:00:52

passed test:python3.6 #570396

00:00:49

passed test:python3.7 #570397

00:00:47