Bump nokogiri from 1.8.5 to 1.9.1

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.8.5 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@v1.8.5...v1.9.1)

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

Gemfile.lock

@@ -180,7 +180,7 @@ GEM
     mime-types (2.99.3)
     mimemagic (0.3.2)
     mini_mime (1.0.1)
-    mini_portile2 (2.3.0)
+    mini_portile2 (2.4.0)
     minitest (5.11.3)
     minitest-slow_test (0.2.0)
       minitest (>= 5.0)
@@ -194,8 +194,8 @@ GEM
     multipart-post (2.0.0)
     netrc (0.11.0)
     nio4r (2.3.1)
-    nokogiri (1.8.5)
-      mini_portile2 (~> 2.3.0)
+    nokogiri (1.9.1)
+      mini_portile2 (~> 2.4.0)
     oauth (0.5.4)
     oauth2 (1.4.1)
       faraday (>= 0.8, < 0.16.0)