Clean up Gemfile and _config.yml

Drop plugins that are included in github-pages by default, drop stuff
that isn't needed/used (minima, Windows-specific gems), drop baseurl.

The thinning of Gemfile may actually be necessary, as bundler's
dependency resolution sometimes installs a horribly old version of
github-pages when it starts the resolution with jekyll-default-layout,
as github-pages depends on an older version of jekyll-default-layout,
and its resolution algorithm then goes all crazy.

Gemfile

@@ -1,15 +1,3 @@
 source "https://rubygems.org"
-gem "minima", "~> 2.5"
 
 gem "github-pages", group: :jekyll_plugins
-
-group :jekyll_plugins do
-  gem "jekyll-default-layout"
-end
-
-platforms :mingw, :x64_mingw, :mswin, :jruby do
-  gem "tzinfo", "~> 1.2"
-  gem "tzinfo-data"
-end
-
-gem "wdm", "~> 0.1.1", :platforms => [:mingw, :x64_mingw, :mswin]

_config.yml

@@ -1,4 +1 @@
-baseurl: ""
 url: "https://xmonad.org"
-plugins:
-  - jekyll-default-layout