Bump nokogiri from 1.8.2 to 1.13.4

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.8.2 to 1.13.4.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/v1.13.4/CHANGELOG.md)
- [Commits](sparklemotion/nokogiri@v1.8.2...v1.13.4)

---
updated-dependencies:
- dependency-name: nokogiri
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

Gemfile

@@ -8,4 +8,4 @@ gem 'middleman-autoprefixer', '~> 2.7.0'
 gem 'middleman-sprockets', '~> 4.1.0'
 gem 'rouge', '~> 2.0.5'
 gem 'redcarpet', '~> 3.4.0'
-gem 'nokogiri', '~> 1.8.2'
+gem 'nokogiri', '~> 1.13.4'

Gemfile.lock

@@ -79,10 +79,11 @@ GEM
     middleman-syntax (3.0.0)
       middleman-core (>= 3.2)
       rouge (~> 2.0)
-    mini_portile2 (2.3.0)
+    mini_portile2 (2.8.0)
     minitest (5.10.1)
-    nokogiri (1.8.2)
-      mini_portile2 (~> 2.3.0)
+    nokogiri (1.13.4)
+      mini_portile2 (~> 2.8.0)
+      racc (~> 1.4)
     padrino-helpers (0.13.3.3)
       i18n (~> 0.6, >= 0.6.7)
       padrino-support (= 0.13.3.3)
@@ -91,6 +92,7 @@ GEM
       activesupport (>= 3.1)
     parallel (1.10.0)
     public_suffix (2.0.5)
+    racc (1.6.0)
     rack (2.0.5)
     rb-fsevent (0.9.8)
     rb-inotify (0.9.8)
@@ -118,7 +120,7 @@ DEPENDENCIES
   middleman-autoprefixer (~> 2.7.0)
   middleman-sprockets (~> 4.1.0)
   middleman-syntax (~> 3.0.0)
-  nokogiri (~> 1.8.2)
+  nokogiri (~> 1.13.4)
   redcarpet (~> 3.4.0)
   rouge (~> 2.0.5)