Increases strictness of explicit i18n dependency

i18n v0.6.7 introduces I18n.enforce_available_locales=, which Padrino
depends on. Earlier versions of the i18n gem that are on the 0.6 line
will satisfy the gemspec dependency but will not allow the tests to
pass since the gem lacks that method.
Merge pull request #1533 from fj/topic/specify-i18n-dependency

Increase strictness of explicit i18n dependency