Fix Relation#count deprecation warning

The following warnings appear when testing Tolk with Rails ~>4.0.4:

```
DEPRECATION WARNING: Relation#calculate with finder options is deprecated. Please build a scope and then call calculate on it instead. (called from has_updated_translations? at tolk/app/models/tolk/locale.rb:78)
DEPRECATION WARNING: The :distinct option for `Relation#count` is deprecated. Please use `Relation#distinct` instead. (eg. `relation.distinct.count`). (called from has_updated_translations? at tolk/app/models/tolk/locale.rb:78)
```