[Security] Bump nokogiri from 1.6.6.2 to 1.9.1

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.6.6.2 to 1.9.1. **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@v1.6.6.2...v1.9.1)

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

Author: dependabot-support

Gemfile.lock

@@ -156,7 +156,7 @@ GEM
       mime-types (>= 1.16, < 3)
     method_source (0.8.2)
     mime-types (2.6.2)
-    mini_portile (0.6.2)
+    mini_portile2 (2.4.0)
     minitest (4.7.5)
     mongoid (4.0.2)
       activemodel (~> 4.0)
@@ -178,8 +178,8 @@ GEM
       moped
       newrelic_rpm (~> 3.11)
     newrelic_rpm (3.13.2.302)
-    nokogiri (1.6.6.2)
-      mini_portile (~> 0.6.0)
+    nokogiri (1.9.1)
+      mini_portile2 (~> 2.4.0)
     oauth2 (1.0.0)
       faraday (>= 0.8, < 0.10)
       jwt (~> 1.0)