Additional install step for Ubuntu on Windows

[argium]

Before submitting an issue, please be sure to

• [x] Read the contributing instructions
• [x] Update to the latest Gem version (run bundle update github-pages)

This issue affects

• [x] Building sites locally

The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting seeking support in other forums.

What did you do (e.g., steps to reproduce)

1. Followed these instructions to get Jekyll building in Ubuntu on Windows (WSL)
2. Commented out jekyll in the gem file.
3. Uncommented out github-pages in the gem file
4. Ran bundle update

What did you expect to happen?

Bundle should succeed.

What happened instead?

Fetching nokogiri 1.8.4
Installing nokogiri 1.8.4 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /home/rifuller/gems/gems/nokogiri-1.8.4/ext/nokogiri
/usr/bin/ruby2.5 -r ./siteconf20180731-25549-127mr9l.rb extconf.rb
checking if the C compiler accepts ... yes
Building nokogiri using packaged libraries.
Using mini_portile version 2.3.0
checking for gzdopen() in -lz... no
zlib is missing; necessary for building libxml2
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Additional information

sudo apt-get install zlib1g-dev is required which isn't installed by default or by jekyll's instructions. There should probably be an extra step in the documentation.

• Link to the live site (if applicable): N/A
• Link to the source repo (if applicable): N/A

[tianvan]

same question @rifuller