Revert i18n pin, allow 0.7.x
The pin on the i18n gem in #8791 can be reverted, allowing i18n 0.7.0 (and therefore rails-i18n 4.0.6+ with newer translations) to be installed.
It was pinned as older versions in use didn't have the enforce_available_locales setting and 0.7.0 defaulted it to true. #7230 set enforce_available_locales to false, but the pin wasn't lifted in the merged version.
fixes #13256 - unpin i18n gem to get 0.7.x
Allows rails-i18n to be updated to the latest 4.0.x release.