[ruby/rdoc] Fix failing test by adding gettext as a development dependency

The failing test was:

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rdoc/test/rdoc/test_rdoc_i18n_locale.rb:35:in `rescue in test_load_existent_po'
Omission: gettext gem is not found [test_load_existent_po(TestRDocI18nLocale)]¶

https://github.com/ruby/rdoc/commit/e5f85839e1