Set default_locale for custom I18n config

Otherwise, the `with_locale` block will try to set back any locale set
by the host app, which might or might not be :en, leading to an
`I18n::InvalidLocale` error.

Addresses #219.

Author: rosa

lib/mission_control/jobs/i18n_config.rb

@@ -2,4 +2,8 @@ class MissionControl::Jobs::I18nConfig < ::I18n::Config
   def available_locales
     [ :en ]
   end
+
+  def default_locale
+    :en
+  end
 end