Bump nokogiri from 1.6.7.2 to 1.13.4

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.6.7.2 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@v1.6.7.2...v1.13.4)

---
updated-dependencies:
- dependency-name: nokogiri
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Gemfile.lock

@@ -1,4 +1,3 @@
-
 GEM
   remote: https://rubygems.org/
   specs:
@@ -13,7 +12,7 @@ GEM
     colored (1.2)
     ethon (0.8.1)
       ffi (>= 1.3.0)
-    ffi (1.9.10)
+    ffi (1.15.5)
     html-proofer (2.6.4)
       activesupport (~> 4.2)
       addressable (~> 2.3)
@@ -46,14 +45,16 @@ GEM
       rb-fsevent (>= 0.9.3)
       rb-inotify (>= 0.9)
     mercenary (0.3.5)
-    mini_portile2 (2.0.0)
+    mini_portile2 (2.8.0)
     minitest (5.8.4)
-    nokogiri (1.6.7.2)
-      mini_portile2 (~> 2.0.0.rc2)
+    nokogiri (1.13.4)
+      mini_portile2 (~> 2.8.0)
+      racc (~> 1.4)
     parallel (1.6.1)
+    racc (1.6.0)
     rb-fsevent (0.9.7)
-    rb-inotify (0.9.6)
-      ffi (>= 0.5.0)
+    rb-inotify (0.10.1)
+      ffi (~> 1.0)
     rouge (1.10.1)
     safe_yaml (1.0.4)
     sass (3.4.21)