Bump nokogiri from 1.8.1 to 1.13.4 in /app

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.8.1 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@v1.8.1...v1.13.4)

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

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

Author: dependabot[bot]

app/Gemfile.lock

@@ -89,15 +89,17 @@ GEM
       mini_mime (>= 0.1.1)
     method_source (0.9.0)
     mini_mime (1.0.0)
-    mini_portile2 (2.3.0)
+    mini_portile2 (2.8.0)
     minitest (5.11.1)
     multi_json (1.13.1)
     nio4r (2.2.0)
-    nokogiri (1.8.1)
-      mini_portile2 (~> 2.3.0)
+    nokogiri (1.13.4)
+      mini_portile2 (~> 2.8.0)
+      racc (~> 1.4)
     pg (0.21.0)
     public_suffix (3.0.1)
     puma (3.11.0)
+    racc (1.6.0)
     rack (2.0.3)
     rack-cors (1.0.5)
       rack (>= 1.6.0)