[Security] Bump nokogiri from 1.8.2 to 1.8.4

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.8.2 to 1.8.4. **This update includes security fixes.**
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md)
- [Commits](sparklemotion/nokogiri@v1.8.2...v1.8.4)

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

Gemfile.lock

@@ -137,7 +137,7 @@ GEM
     msgpack (1.2.4)
     multi_json (1.13.1)
     nio4r (2.3.0)
-    nokogiri (1.8.2)
+    nokogiri (1.8.4)
       mini_portile2 (~> 2.3.0)
     parallel (1.12.1)
     parser (2.5.1.0)