Add stretch build

THis is purely copy-pasta from the Jessie build.
11 jobs for add/debian-stretch in 7 minutes and 30 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #1973
lint-dockerfile

00:01:29

passed #1974
lint-shell-script

00:01:25

 
  Deploy
passed #1977
build-debian-jessie-build-dev

00:06:00

passed #1976
build-debian-stretch-build-dev

00:02:40

passed #1975
build-golang-dev

00:02:34

passed #1983
allowed to fail
build-php-5.3-dev

00:04:03

passed #1982
allowed to fail
build-php-5.6-dev

00:03:53

passed #1981
allowed to fail
build-php-7.0-dev

00:03:54

passed #1980
build-php-7.1-dev

00:04:01

passed #1979
build-php-7.2-dev

00:04:05

passed #1978
build-php-7.3-dev

00:04:51