Use an updated version of ruby

With this configuration, it always uses an updated version of ruby and bundler
is the one which manages version of the dependencies, hence working in all
environments. Before, dependencies could not be satisfied in updated
environments because of bundler being too new.

Gemfile

@@ -1,9 +1,30 @@
 source "https://rubygems.org"
-ruby RUBY_VERSION
-
+# Hello! This is where you manage which Jekyll version is used to run.
+# When you want to use a different version, change it below, save the
+# file and run `bundle install`. Run Jekyll with `bundle exec`, like so:
+#
+#     bundle exec jekyll serve
+#
 # This will help ensure the proper Jekyll version is running.
-gem "jekyll", "3.4.0"
+# Happy Jekylling!
+gem "jekyll", "~> 4.0.0"
+# This is the default theme for new Jekyll sites. You may change this to anything you like.
+gem "minima", "~> 2.5"
+# If you want to use GitHub Pages, remove the "gem "jekyll"" above and
+# uncomment the line below. To upgrade, run `bundle update github-pages`.
+# gem "github-pages", group: :jekyll_plugins
+# If you have any plugins, put them here!
+group :jekyll_plugins do
+  gem "jekyll-feed", "~> 0.12"
+end
 
-# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
-gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
+# Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
+# and associated library.
+install_if -> { RUBY_PLATFORM =~ %r!mingw|mswin|java! } do
+  gem "tzinfo", "~> 1.2"
+  gem "tzinfo-data"
+end
+
+# Performance-booster for watching directories on Windows
+gem "wdm", "~> 0.1.1", :install_if => Gem.win_platform?