Add missing windows extensions

When running on Windows, wdm is required in order to properly do
incremental updates and regeneration.

Fix dependency on timezone to match docs

See https://jekyllrb.com/docs/installation/windows/#time-zone-management

Gemfile

@@ -2,5 +2,14 @@
 
 source "https://rubygems.org"
 
+gem "wdm", "~> 0.1.1", :install_if => Gem.win_platform?
+
+# Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
+# and associated library.
+platforms :mingw, :x64_mingw, :mswin, :jruby do
+    gem "tzinfo", ">= 1", "< 3"
+    gem "tzinfo-data"
+  end
+
 gemspec