Release 2.4.0

• New feature: Initial support for GCM/FCM "notification" pushes (Documented in PR #185).
  uniqush.notification.gcm and uniqush.notification.fcm can be used
  as fields for /push, with the JSON blob to send GCM/FCM for the
  optional "notification" message.
  "notification" messages will let GCM/FCM display the notification for you.
• Maintenance: Change from https://android.googleapis.com/gcm/send to
  https://gcm-http.googleapis.com/gcm/send (equivalent endpoints)
• Maintenance: Bump go version used to compile releases
• Maintenance: go 1.9+ is recommended for compiling and testing
• Bug fix: Improve logging subscriber name in failed API requests.