Use apt_repository module instead of handling apt-get update ourself

The apt_repository module already handles apt-get updating after install of a
repo, so use that instead of handling it ourself. Also, ansible-lint didn't
like our approach.

The change also requires Ansible >= 2.1 due to the filename option to
apt_repository.

files/jessie-backports.list

@@ -1,2 +0,0 @@
-deb http://ftp.de.debian.org/debian jessie-backports main
-deb-src http://ftp.de.debian.org/debian jessie-backports main

tasks/main.yml

@@ -1,7 +1,9 @@
 - name: install jessie-backports apt list
-  copy: src=jessie-backports.list dest=/etc/apt/sources.list.d/
-  register: jessie_backports_list
+  apt_repository:
+    filename: jessie-backports
+    repo: "deb http://ftp.de.debian.org/debian jessie-backports main"
 
-- name: apt-get update
-  apt: update_cache=yes
-  when: jessie_backports_list|changed
+- name: install jessie-backports sources apt list
+  apt_repository:
+    filename: jessie-backports
+    repo: "deb-src http://ftp.de.debian.org/debian jessie-backports main"