Bump nokogiri from 1.6.0 to 1.13.4 in /sample_app

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.6.0 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@v1.6.0...v1.13.4)

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

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

Author: dependabot[bot]

sample_app/Gemfile.lock

@@ -75,17 +75,19 @@ GEM
       treetop (~> 1.4.8)
     method_source (0.8.1)
     mime-types (1.23)
-    mini_portile (0.5.1)
+    mini_portile2 (2.8.0)
     multi_json (1.7.7)
     multi_test (0.0.1)
-    nokogiri (1.6.0)
-      mini_portile (~> 0.5.0)
+    nokogiri (1.13.4)
+      mini_portile2 (~> 2.8.0)
+      racc (~> 1.4)
     pg (0.15.1)
     polyglot (0.3.3)
     pry (0.9.12.2)
       coderay (~> 1.0.5)
       method_source (~> 0.8)
       slop (~> 3.4)
+    racc (1.6.0)
     rack (1.4.5)
     rack-cache (1.2)
       rack (>= 0.4)