Gemfile: Resolve "uninitialized constant TZInfo:Timezone (NameError)"

that may be seen on Windows.

Thanks to @Andre601 for sharing his solution on
tzinfo/tzinfo#144 (comment)

docs/Gemfile

@@ -2,10 +2,16 @@
 
 source "https://rubygems.org"
 
+gem 'wdm', '~> 0.1.1', :install_if => Gem.win_platform?
+
+gem "webrick"
+
+platforms :mingw, :x64_mingw, :mswin, :jruby do
+  gem 'tzinfo', '>= 1', '< 3'
+  gem 'tzinfo-data'
+end
+
 group :jekyll_plugins do
   gem "github-pages"
   gem "jekyll-github-metadata"
-  gem 'tzinfo-data'
-  gem "webrick"
-  gem 'wdm', '>= 0.1.0' if Gem.win_platform?
 end