I ran into this problem on a fresh install of Ubuntu 18.04.3 LTS. I had not installed development tools yet, so I found this which prompted me to add:

• ruby-dev
• gcc
• libffi-dev
• make

That got the gems building, but it kept giving me an error after build:

checking for gzdopen() in -lz... no
zlib is missing; necessary for building libxml2

well shit.

 sudo apt install zlib
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package zlib

According to this I then ran:

sudo apt-get install zlib1g-dev

And boom.