Fix error when including Fallbacks on non-Simple backend

Commit 12aa0f0d0fc1822d6825d81bb9cdfdc4eb325dfd introduced a bug, where
if `Fallbacks` module is included in a class that doesnt define
`translations` the code would fail. This is a pretty common
scenario, for instance when the backend is a `Chain`. Also this was
pretty common case to fail in rails, as its include Fallbacks in the
I18n.backend, like this:
```
I18n.backend.class.send(:include, I18n::Backend::Fallbacks)
```

This stops using the `translations` method in the fallbacks, and instead
ignores `I18n::InvalidLocale` errors.

[fixes #238]
[fixes #258]
[fixes #259]