Add wdm gem for Windows.

Gemfile

@@ -4,4 +4,5 @@ group :development, :test do
   gem 'jekyll', '~> 3.8.5'
   gem 'jekyll-redirect-from', '~> 0.14.0'
   gem 'jekyll-sitemap', '~> 1.2.0'
+  gem 'wdm', '~> 0.1.1', :install_if => Gem.win_platform?
 end

Gemfile.lock

@@ -58,6 +58,7 @@ GEM
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
+    wdm (0.1.1)
 
 PLATFORMS
   ruby
@@ -67,6 +68,7 @@ DEPENDENCIES
   jekyll (~> 3.8.5)
   jekyll-redirect-from (~> 0.14.0)
   jekyll-sitemap (~> 1.2.0)
+  wdm (~> 0.1.1)
 
 BUNDLED WITH
    1.17.3