Fix failing Debian tests.

.travis.yml

@@ -10,6 +10,7 @@ env:
     - MOLECULE_DISTRO: fedora27
     - MOLECULE_DISTRO: ubuntu1804
     - MOLECULE_DISTRO: debian9
+    - MOLECULE_DISTRO: debian8
 
 install:
   # Install test dependencies.

defaults/main.yml

@@ -0,0 +1,2 @@
+---
+ansible_default_release: ""

tasks/setup-Debian.yml

@@ -6,12 +6,18 @@
       {{ ansible_distribution_release }}-backports main'
     state: present
     filename: "{{ ansible_distribution_release }}_backports"
+  when: ansible_distribution_version < 9
 
 - name: Update apt cache.
   apt: update_cache=yes cache_valid_time=86400
 
+- name: Set the default_release option for older Debian versions.
+  set_fact:
+    ansible_default_release: "{{ ansible_distribution_release }}-backports"
+  when: ansible_distribution_version < 9
+
 - name: Install Ansible.
   apt:
     name: ansible
     state: present
-    default_release: "{{ ansible_distribution_release }}-backports"
+    default_release: "{{ ansible_default_release }}"