Install lxc from standard apt repository rather than backports

roles/gitian/tasks/main.yml

@@ -24,6 +24,7 @@
     - gnupg2
     - kpartx
     - lintian
+    - lxc
     - make
     - python-cheetah
     - qemu-utils
@@ -107,19 +108,6 @@
     group: root
     mode: "0644"
 
-- name: Set up jessie-backports.
-  apt_repository:
-    repo: 'deb http://httpredir.debian.org/debian/ jessie-backports main'
-    state: present
-
-- name: Install updated version of LXC.
-  apt:
-    name: lxc
-    state: latest
-    default_release: jessie-backports
-    update_cache: yes
-    cache_valid_time: 3600
-
 - name: Clone git repository for Gitian builder.
   git:
     repo: "{{ gitian_builder_url }}"