Upgrade to kramdown v2 and add kramdown-parser-gfm to allow GFM input

lib/github-pages/dependencies.rb

@@ -11,7 +11,8 @@ module GitHubPages
       "jekyll-sass-converter" => "1.5.2",
 
       # Converters
-      "kramdown" => "1.17.0",
+      "kramdown" => "2.3.0",
+      "kramdown-parser-gfm" => "1.1.0",
       "jekyll-commonmark-ghpages" => "0.1.6",
 
       # Misc

spec/fixtures/kramdown-parser-gfm.md

@@ -0,0 +1,6 @@
+---
+---
+
+# Test
+
+~~Nope~~Yes

spec/github-pages/dependencies_spec.rb

@@ -4,7 +4,7 @@ require "spec_helper"
 
 describe(GitHubPages::Dependencies) do
   CORE_DEPENDENCIES = %w(
-    jekyll kramdown liquid rouge jekyll-sass-converter
+    jekyll kramdown kramdown-parser-gfm liquid rouge jekyll-sass-converter
     github-pages-health-check
   ).freeze
   PLUGINS = described_class::VERSIONS.keys - CORE_DEPENDENCIES

spec/github-pages/integration_spec.rb

@@ -95,6 +95,12 @@ RSpec.describe "Pages Gem Integration spec" do
       end
     end
 
+    context "kramdown-parser-gfm" do
+      it "converts GFM to HTML" do
+        expect(contents).to match("<del>Nope</del>Yes")
+      end
+    end
+
     context "liquid" do
       it "renders liquid templates" do
         expect(contents).to match("Value of foo: bar")