Bump fluentd from 1.8.1 to 1.9.0

Bumps [fluentd](https://github.com/fluent/fluentd) from 1.8.1 to 1.9.0.
- [Release notes](https://github.com/fluent/fluentd/releases)
- [Changelog](https://github.com/fluent/fluentd/blob/master/CHANGELOG.md)
- [Commits](fluent/fluentd@v1.8.1...v1.9.0)

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

Gemfile.lock

@@ -11,13 +11,11 @@ GEM
       thor (>= 0.19.4, < 2.0)
       tins (~> 1.6)
     diff-lcs (1.3)
-    dig_rb (1.0.1)
     docile (1.3.1)
-    fluentd (1.8.1)
+    fluentd (1.9.0)
       cool.io (>= 1.4.5, < 2.0.0)
-      dig_rb (~> 1.0.0)
       http_parser.rb (>= 0.5.1, < 0.7.0)
-      msgpack (>= 1.2.0, < 2.0.0)
+      msgpack (>= 1.3.1, < 2.0.0)
       serverengine (>= 2.0.4, < 3.0.0)
       sigdump (~> 0.2.2)
       strptime (>= 0.2.2, < 1.0.0)