Supress warning about method redifinition

In i18n gem, the following methods are defined.
- `Hash#except`
- `Hash#slice`

But if there are defined already, i18n skips these definitions.
So these definition by `active_support` are required before `require 'i18n'`.
Merge pull request #9240 from tricknotes/supress-warning

Supress warning about method redifinition