[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](https://github.com/sparklemotion/nokogiri/compare/v1.8.2...v1.8.4)

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

Gemfile.lock

@@ -273,7 +273,7 @@ GEM
       net-ssh (>= 2.6.5)
     net-ssh (4.2.0)
     newrelic_rpm (4.1.0.333)
-    nokogiri (1.8.2)
+    nokogiri (1.8.4)
       mini_portile2 (~> 2.3.0)
     nori (2.6.0)
     oauth (0.5.3)