Add rng-tools to the list of packages to install

This was seen to fix an issue where gpg2 hung waiting for entropy when
generating keys
https://chat.zcashcommunity.com/channel/zcash-dev?msg=AkSCfLbihcgtECiig

dependency_install_steps_by_platform/Ubuntu_18.04.x.md

@@ -11,10 +11,10 @@ Description:	Ubuntu 18.04 LTS
 
 
 
-## Install Git, VirtualBox, Ansible, and GnuPG
+## Install Git, VirtualBox, Ansible, GnuPG, and rng-tools
 
 ```
-$ sudo apt install git virtualbox ansible gnupg2
+$ sudo apt install git virtualbox ansible gnupg2 rng-tools
 ```