Push notification passphrase

[avinoth]

Support Push notification passphrase as config attribute.
Merge last-modified header feature from #26

[frozon]

LGTM if you could also review @lgleasain

[rmosolgo]

👍 I was getting an error from apple for not including this header, but updating to this branch worked for me.

I also ran into the passphrase issue, but I worked around it by using Grocer's notification API directly:

    pusher = Grocer.pusher(
      certificate: Passbook.notification_cert,
      passphrase:  Passbook.p12_password,
      gateway: Rails.env.test? ? "localhost" : "gateway.push.apple.com",
    )

    PassbookDevice.find_each do |device|
      notification = Grocer::PassbookNotification.new(device_token: device.push_token)
      pusher.push(notification)
    end

Thanks for pushing this fix, I hope it can be merged soon!

[avinoth]

@frozon Changed the way empty passphrase is handled, brought down the version bump and squashed the commits. 👍

[frozon]

Thanks for your work on it