drop Debian Stretch support

Fixes #1194

installer/lib/requirements-debian-stretch.apt

@@ -1,59 +0,0 @@
-apache2
-coreutils
-curl
-ecasound
-flac
-git
-gstreamer1.0-plugins-bad
-gstreamer1.0-plugins-good
-gstreamer1.0-plugins-ugly
-icecast2
-lame
-libao-ocaml
-libapache2-mod-php7.0
-libcamomile-ocaml-data
-libfaad2
-libmad-ocaml
-libopus0
-libportaudio2
-libpulse0
-libpq-dev
-libsamplerate0
-libsoundtouch-ocaml
-libtaglib-ocaml
-libvo-aacenc0
-liquidsoap
-lsb-release
-lsb-release
-lsof
-mpg123
-patch
-php7.0
-php7.0-bcmath
-php7.0-curl
-php7.0-dev
-php7.0-gd
-php7.0-mbstring
-php7.0-pgsql
-php-amqplib
-php-apcu
-php-pear
-postgresql
-postgresql-client
-pwgen
-python3
-python3-gst-1.0
-python3-pika
-python3-pip
-python3-virtualenv
-python3-cairo
-rabbitmq-server
-systemd-sysv
-unzip
-uwsgi
-uwsgi-plugin-python3
-vorbisgain
-vorbis-tools
-vorbis-tools
-xmlstarlet
-zip