Update installation.md for systemd distributions

doc/install/installation.md

@@ -110,7 +110,7 @@ Install the required packages (needed to compile Ruby and native extensions to R
 sudo apt-get install -y build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libre2-dev \
   libreadline-dev libncurses5-dev libffi-dev curl openssh-server checkinstall libxml2-dev \
   libxslt-dev libcurl4-openssl-dev libicu-dev logrotate rsync python-docutils pkg-config cmake \
-  runit
+  runit-systemd
 ```
 
 Ubuntu 14.04 (Trusty Tahr) doesn't have the `libre2-dev` package available, but