Fix docker_image option for ansible < 2.8

roles/matrix-synapse/tasks/goofys/setup_install.yml

@@ -1,7 +1,7 @@
 - name: Ensure Goofys Docker image is pulled
   docker_image:
     name: "{{ matrix_s3_goofys_docker_image }}"
-    source: "pull"
+    source: "{{ 'pull' if ansible_version.major > 2 or ansible_version.minor > 7 else omit }}"
 
 # This will throw a Permission Denied error if already mounted
 - name: Check Matrix Goofys external storage mountpoint path

roles/matrix-synapse/tasks/synapse/setup_install.yml

@@ -21,7 +21,7 @@
 - name: Ensure Synapse Docker image is pulled
   docker_image:
     name: "{{ matrix_synapse_docker_image }}"
-    source: "pull"
+    source: "{{ 'pull' if ansible_version.major > 2 or ansible_version.minor > 7 else omit }}"
 
 - name: Check if a Synapse signing key exists
   stat: