Use id instead of uid for VK provider

[anaumov]

VK has changed the API and it returns user_info in such way

{
  email: 'someone@vk.com',
  id: 12300321
}

Therefore user_hash[:user_info]['uid'] is nil and sorcery does not create authentication for user. This patch should fix this.

Not sure how to add spec for this case. Let me know if you have any questions.

[anaumov]

@athix can you take a look?

[joshbuker]

I'm currently slammed, but I'll try to get caught up with Sorcery as soon as possible.