Fix an issue with setting I18n.config.enforce_available_locales

currently setting this config to false will not stick due to
the memoize overwriting it back to nil. this probaly works ok
in many circumstances because of falsy nil, but annoyingly it
does not prevent the deprecation warning that comes along
with having the setting unconfigured.
Merge pull request #223 from worace/allow-false-as-enforce_available_locales-setting

Fix an issue with setting I18n.config.enforce_available_locales