Merge pull request #3473 from edx/bilalahmad99/forum-16.04

forums on 16.04

docker/build/forum/Dockerfile

-FROM edxops/precise-common:latest
+FROM edxops/xenial-common:latest
 MAINTAINER edxops
 
 WORKDIR /edx/app/edx_ansible/edx_ansible/docker/plays

playbooks/roles/rbenv/tasks/main.yml

@@ -55,7 +55,7 @@
     - install:base
 
 - name: install build depends
-  apt: pkg={{ ",".join(rbenv_debian_pkgs) }} state=present install_recommends=no
+  apt: pkg={{ ",".join(rbenv_debian_pkgs) }} update_cache=yes state=present install_recommends=no
   with_items: rbenv_debian_pkgs
   tags:
     - install