Incompatible dependencies with github-pages?

[tj-cahill]

Wanted to test out this theme on my GitHub Pages site, and on trying to install the Ruby gem for the theme using the method described in the README (by modifying the Gemfile and using bundler), I get the following error:

Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Could not find compatible versions

Because github-pages >= 228 depends on jekyll = 3.9.3
  and every version of academic-jekyll-theme depends on jekyll ~> 4.2,
  github-pages >= 228 is incompatible with academic-jekyll-theme >= 0.
So, because Gemfile depends on academic-jekyll-theme >= 0
  and Gemfile depends on github-pages ~> 228,
  version solving has failed.

My Gemfile when trying this was as follows:

source "https://rubygems.org"

# gem "jekyll", "~> 4.3.2"

# gem "minima", "~> 2.5"
gem "academic-jekyll-theme"

gem "github-pages", "~> 228", group: :jekyll_plugins

group :jekyll_plugins do
  gem "jekyll-feed", "~> 0.12"
end

platforms :mingw, :x64_mingw, :mswin, :jruby do
  gem "tzinfo", ">= 1", "< 3"
  gem "tzinfo-data"
end

gem "wdm", "~> 0.1.1", :platforms => [:mingw, :x64_mingw, :mswin]
gem "http_parser.rb", "~> 0.6.0", :platforms => [:jruby]
gem "webrick", "~> 1.8"