Let AspectIdentifier class comply with the AspectToken protocol.

[Lision]

Hi~ @steipete

When i use Aspects, i write some code like this:

if ([token conformsToProtocol:@protocol(AspectToken)]) {
    [token remove];
}

But the [token remove]; is not implemented. I find this because AspectIdentifier class do not comply with the AspectToken protocol. So, I create this pull request. I sincerely hope you can merge it.

Then I update the project setting to Xcode 9 to pass the unit tests.