Log Message:
Update ruby-i18n to 0.8.4.

0.8.4					2017/05/31

* Revert "Don't allow nil to be submitted as a key to i18n.translate()"

0.8.3					2017/5/30

* I18n::Gettext#plural_keys will now return a hash from Gettext if no
  arguments are provided - #122
* Fixed a bug where passing false to translate would not translate that value
  - #367

0.8.2					2017/5/30

* Do not allow nil to be passed to translate - #236