update signature to translations method of translateProvider

accordingly to angular-translate source (https://github.com/angular-translate/angular-translate/blob/master/src/service/translate.js#L235-L252) `translations()` method returns translation table when no params or only key was passed.

angular-translate/angular-translate.d.ts

@@ -72,7 +72,7 @@ declare namespace angular.translate {
     }
 
     interface ITranslateProvider extends angular.IServiceProvider {
-        translations(): ITranslationTable;
+        translations(key?: string): ITranslationTable;
         translations(key: string, translationTable: ITranslationTable): ITranslateProvider;
         cloakClassName(): string;
         cloakClassName(name: string): ITranslateProvider;