Set I18n.enforce_available_locales=true to fix deprecation warning.
Don't set enforce_available_locales for older version of i18n.

Versions of the i18n gem < 0.6.6 don't have this option.
Merge pull request #832 from mbrictson/fix-i18n-deprecation

Set I18n.enforce_available_locales=true to fix deprecation warning