Switch to Redcarpet for markdown parsing

kramdown seems to be having issues with certain HTML elements.
I was specifically hung up on gz/question/132 but it seems to be a known
issue gettalong/kramdown#766

Gemfile

@@ -4,8 +4,7 @@ source 'https://rubygems.org'
 
 gem 'nanoc', '4.12'
 gem 'erubis', '2.7.0'
-gem 'kramdown', '2.4.0'
-gem 'kramdown-parser-gfm', '1.1.0'
+gem 'redcarpet'
 gem 'json'
 
 group :development do

Gemfile.lock

@@ -18,9 +18,8 @@ GEM
     erubis (2.7.0)
     hamster (3.0.0)
       concurrent-ruby (~> 1.0)
+    json (2.7.1)
     json_schema (0.21.0)
-    kramdown (2.4.0)
-      rexml
     nanoc (4.12.0)
       addressable (~> 2.5)
       colored (~> 1.2)
@@ -62,8 +61,8 @@ GEM
     rackup (2.1.0)
       rack (>= 3)
       webrick (~> 1.8)
+    redcarpet (3.6.0)
     ref (2.0.0)
-    rexml (3.2.6)
     slow_enumerator_tools (1.1.0)
     tomlrb (1.3.0)
     tty-color (0.6.0)
@@ -80,8 +79,9 @@ PLATFORMS
 DEPENDENCIES
   adsf (= 1.4.8)
   erubis (= 2.7.0)
-  kramdown (= 2.4.0)
+  json
   nanoc (= 4.12)
+  redcarpet
 
 BUNDLED WITH
    2.4.22

Rules

@@ -18,7 +18,8 @@ compile '/**/*.html' do
 end
 
 compile '/questions/**/*.md' do
-  filter :kramdown, input: "GFM"
+  #filter :kramdown, input: "GFM"
+  filter :redcarpet
   layout '/default.*'
 
   write item.identifier.without_ext + '/index.html'