Fix i18n validation warning

According to StackOverflow[1], we need to set this explicitly in order to avoid
this warning:

  [deprecated] I18n.enforce_available_locales will default to true in the
  future. If you really want to skip validation of your locale you can set
  I18n.enforce_available_locales = false to avoid this message.

We set it to `true` to future-proof our apps, since that's what the default will
be in the future.

1: http://stackoverflow.com/questions/20361428/rails-i18n-validation-deprecation-warning