Bump nokogiri from 1.8.0 to 1.9.1

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.8.0 to 1.9.1.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/master/CHANGELOG.md)
- [Commits](sparklemotion/nokogiri@v1.8.0...v1.9.1)

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

Gemfile.lock

@@ -452,7 +452,7 @@ GEM
     metaclass (0.0.4)
     method_source (0.9.0)
     mime-types (1.25.1)
-    mini_portile2 (2.2.0)
+    mini_portile2 (2.4.0)
     mocha (1.3.0)
       metaclass (~> 0.0.1)
     multi_json (1.12.2)
@@ -464,8 +464,8 @@ GEM
     nenv (0.3.0)
     net-http-persistent (2.9.4)
     netrc (0.11.0)
-    nokogiri (1.8.0)
-      mini_portile2 (~> 2.2.0)
+    nokogiri (1.9.1)
+      mini_portile2 (~> 2.4.0)
     notiffany (0.1.1)
       nenv (~> 0.1)
       shellany (~> 0.0)