Add a task to find missing translations

Adds [i18n-tasks](https://github.com/glebm/i18n-tasks) to find missing translations.

```bash
$ rake i18n:missing[fr]
Missing translations (6) | i18n-tasks v0.2.9
Types: ✗ key missing, ∅ translation blank, = value same as base value
+--------+------+-----------------------------------------+-----------------------------+
| Locale | Type | i18n Key                                | Base value (en)             |
+--------+------+-----------------------------------------+-----------------------------+
|   fr   |  ∅   | activerecord.errors.messages.other_than | must be other than %{count} |
|   fr   |  ∅   | activerecord.errors.messages.present    | must be blank               |
|   fr   |  ∅   | activemodel.errors.messages.other_than  | must be other than %{count} |
|   fr   |  ∅   | activemodel.errors.messages.present     | must be blank               |
|   fr   |  ∅   | errors.messages.other_than              | must be other than %{count} |
|   fr   |  ∅   | errors.messages.present                 | must be blank               |
+--------+------+-----------------------------------------+-----------------------------+
```