It’s more a reminder than an actual guide in order to install Rocket.Chat behing an Nginc proxy with Ansible and a trusted certificate from Let’s Encrypt on an Ubuntu 15.04 VM (or dedicated server).
I used Ubuntu 15.04 (and not the 14.04 LTS) because the 14.04 only has Python 2.7.6 in the repository whereas the 15.04 has Python 2.7.9 which is the minimum required for some of the libraries used for Let’s Encrypt (and I prefered to use a more recent release of Ubuntu rather than doing some apt-pinning).
I am not at all an expert in any of the technologies and softwares used here so this post may contain some instructions that are not consider best practices, or even mistakes so don't take this post as a master guide, this is just the way I did it and it worked for me so ¯_(ツ)_/¯
It is preferable to have a fresh install of Ubuntu, however it is totally possible to do this in a rather clean install.