Update Jekyll & Kramdown

This picks up a Kramdown security update (in 2.3.1) as well as a
variety of other updates. From manual testing the changes in the
output are:

- code elements (both blocks and inline) now have the additional
  CSS class 'language-plaintext' (which has no effect)
- the CSS classes for inline code highlighting have changed,
  though the appearance is unaffected
- `role` attributes are added to various elements, mostly footnotes

None of these seem to affect the appearance of the docs.

Author: PeterJCLaw

Gemfile

@@ -3,7 +3,10 @@ source 'https://rubygems.org'
 gem 'rake'
 gem 'neat'
 gem 'kramdown'
-gem 'jekyll'
+# jekyll depends on kramdown-parser-gfm, but fails to declare it
+gem 'kramdown-parser-gfm'
+# Pin to the version used on GitHub Pages
+gem 'jekyll', 3.9
 gem 'jekyll-redirect-from'
 
 # Avoid polling on windows

Gemfile.lock

@@ -1,29 +1,29 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.5.2)
-      public_suffix (>= 2.0.2, < 4.0)
+    addressable (2.7.0)
+      public_suffix (>= 2.0.2, < 5.0)
     colorator (1.1.0)
-    concurrent-ruby (1.1.3)
-    em-websocket (0.5.1)
+    concurrent-ruby (1.1.8)
+    em-websocket (0.5.2)
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0.6.0)
     eventmachine (1.2.7)
     eventmachine (1.2.7-x64-mingw32)
-    ffi (1.9.25)
-    ffi (1.9.25-x64-mingw32)
+    ffi (1.15.0)
+    ffi (1.15.0-x64-mingw32)
     forwardable-extended (2.6.0)
     http_parser.rb (0.6.0)
     i18n (0.9.5)
       concurrent-ruby (~> 1.0)
-    jekyll (3.8.5)
+    jekyll (3.9.0)
       addressable (~> 2.4)
       colorator (~> 1.0)
       em-websocket (~> 0.5)
       i18n (~> 0.7)
       jekyll-sass-converter (~> 1.0)
       jekyll-watch (~> 2.0)
-      kramdown (~> 1.14)
+      kramdown (>= 1.17, < 3)
       liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
@@ -33,29 +33,31 @@ GEM
       jekyll (>= 3.3, < 5.0)
     jekyll-sass-converter (1.5.2)
       sass (~> 3.4)
-    jekyll-watch (2.1.2)
+    jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (1.17.0)
-    liquid (4.0.1)
-    listen (3.1.5)
-      rb-fsevent (~> 0.9, >= 0.9.4)
-      rb-inotify (~> 0.9, >= 0.9.7)
-      ruby_dep (~> 1.2)
+    kramdown (2.3.1)
+      rexml
+    kramdown-parser-gfm (1.1.0)
+      kramdown (~> 2.0)
+    liquid (4.0.3)
+    listen (3.5.1)
+      rb-fsevent (~> 0.10, >= 0.10.3)
+      rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
     neat (3.0.0)
       sass (~> 3.4)
       thor (~> 0.19)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (3.0.3)
+    public_suffix (4.0.6)
     rake (12.3.1)
-    rb-fsevent (0.10.3)
-    rb-inotify (0.9.10)
-      ffi (>= 0.5.0, < 2)
-    rouge (3.3.0)
-    ruby_dep (1.5.0)
-    safe_yaml (1.0.4)
-    sass (3.7.2)
+    rb-fsevent (0.10.4)
+    rb-inotify (0.10.1)
+      ffi (~> 1.0)
+    rexml (3.2.4)
+    rouge (3.26.0)
+    safe_yaml (1.0.5)
+    sass (3.7.4)
       sass-listen (~> 4.0.0)
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
@@ -68,12 +70,13 @@ PLATFORMS
   x64-mingw32
 
 DEPENDENCIES
-  jekyll
+  jekyll (= 3.9)
   jekyll-redirect-from
   kramdown
+  kramdown-parser-gfm
   neat
   rake
   wdm (>= 0.1.0)
 
 BUNDLED WITH
-   1.12.4
+   1.17.3