async abilities

[AmilKey]

Hello, the latest version breaks the async abilities

on this line "double negation" returns always true if result for this.valueFor is promise, the same happens in helpers so it is not possible to use with async helpers like (await (can ""))

https://github.com/minutebase/ember-can/blob/master/addon/services/abilities.js#L74

my suggestion is not to use "!!" operator and just return result

  can(abilityString, model, properties) {
    let { propertyName, abilityName } = this.parse(abilityString);
    return this.valueFor(propertyName, abilityName, model, properties);
  }

related #20