fix issue #433

There is a problem (addressed by issue #433) where attributes like `translation-value-name="{{name}}"` result in an empty translation when used inside of an `ng-if` directive.

This problem is caused by `$observe('translate')` and `$watch('translationProperties')` being triggered in reversed order inside of `ng-if`. Hence, the translation is never set as the element content.

This commit adds a second watcher for `translationId` to resolve this problem.