Remove support for go 1.6 (#142)

Removes support for golang 1.6 so we can better support new features
moving forward

Author: sideshow

.travis.yml

@@ -1,7 +1,6 @@
 language: go
 
 go:
-  - 1.6.x
   - 1.7.x
   - 1.8.x
   - 1.9.x

README.md

@@ -8,7 +8,7 @@ APNS/2 is a go package designed for simple, flexible and fast Apple Push Notific
 
 - Uses new Apple APNs HTTP/2 connection
 - Fast - See [notes on speed](https://github.com/sideshow/apns2/wiki/APNS-HTTP-2-Push-Speed)
-- Works with go 1.6 and later
+- Works with go 1.7 and later
 - Supports new Apple Token Based Authentication (JWT)
 - Supports new iOS 10 features such as Collapse IDs, Subtitles and Mutable Notifications
 - Supports persistent connections to APNs