Gemfile.ilm in fluent-plugin-elasticsearch-4.0.5

- old
+ new

@@ -6,5 +6,6 @@
 gem 'simplecov', require: false
 gem 'coveralls', require: false
 gem 'strptime', require: false if RUBY_ENGINE == "ruby" && RUBY_VERSION =~ /^2/
 gem "irb" if RUBY_ENGINE == "ruby" && RUBY_VERSION >= "2.6"
 gem "elasticsearch-xpack"
+gem "oj"
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