add dependency structure

roles/signal/meta/main.yml

@@ -0,0 +1,2 @@
+dependencies:
+  - role: common

roles/signal/tasks/main.yml

@@ -7,11 +7,12 @@
   become: true
   apt_repository:
     repo: "deb [arch=amd64 signed-by=/etc/apt/trusted.gpg.d/signal.asc] https://updates.signal.org/desktop/apt xenial main"
-    state: present
+  register: apt_repository
+- name: Update APT cache
+  apt:
+    update_cache: true
+  when: apt_repository.changed
 - name: Install Signal
   become: true
   apt:
-    state: latest
-    update_cache: yes
-    cache_valid_time: 86400 # One day
     name: signal-desktop