Add provisioning steps for older boxes

conf/vagrant/provisioning/roles/common/tasks/main.yml

@@ -1,4 +1,20 @@
 ---
+
+# These steps ("Update Yarn apt key" and "Remove Duplicity PPA repo") were added to fix issues with
+# updating the apt cache on older boxes. Once the older versions (1.4.0 and below) are no longer supported,
+# these first two steps can be removed.
+
+- name: Common | Update Yarn apt key
+  become: true
+  command: apt-key adv --keyserver keys.gnupg.net --recv-keys 72ECF46A56B4AD39C907BBB71646B01B86E50310
+
+- name: Common | Remove Duplicity PPA repo
+  become: true
+  apt_repository:
+    repo: ppa:duplicity-team
+    state: absent
+  tags: common
+
 - name: Common | Update apt cache
   become: true
   apt: update-cache=yes