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 #570393
build:debian:buster

00:02:02

passed #570392
build:debian:stretch

00:02:04

passed #570391
build:el7

00:01:16

passed #570390
build:tarball

00:00:27

 
  Test
passed #570398
docs

00:01:14

passed #570399
syntax

00:00:53

passed #570394
test:python2.7

00:00:44

passed #570395
test:python3.5

00:00:52

passed #570396
test:python3.6

00:00:49

passed #570397
test:python3.7

00:00:47