improve spanish translation (#333)

Accordingly to a spanish user the translation for validation error "Times
must be explicit" should be "Horas deben ser explícitas". Changed the
template accordingly. Hopefully that also makes sense for other unique
validations.

Closes #311

app/locales/es/translations.js

@@ -150,7 +150,7 @@ export default {
     url: '{{description}} debe de ser una URL valida',
     time: '{{description}} debe de ser un horario valido (p.ej. 10:45)',
     'time.notPartially': 'Partially times are not supported',
-    unique: '{{description}} debe de ser único',
+    unique: '{{description}} deben ser explícitas',
     'unique.name': 'Este nombre ya está usado'
   }
 };