travis: Support recent versions and remove legacy testing

This removes the horribly old 1.5x and replaces it with golang 1.7 which
is apparently still used somewhere.
It also adds tests for the most recent version: 1.11.
Please upgrade or use older releases.

Author: purpleidea

.travis.yml

@@ -5,8 +5,9 @@ services:
 
 language: go
 go:
-  - "1.5.x"
+  - "1.7.x"
   - "1.10.x"
+  - "1.11.x"
 go_import_path: github.com/coreos/go-systemd
 
 env:

README.md

@@ -2,7 +2,7 @@
 
 [![Build Status](https://travis-ci.org/coreos/go-systemd.png?branch=master)](https://travis-ci.org/coreos/go-systemd)
 [![godoc](https://godoc.org/github.com/coreos/go-systemd?status.svg)](http://godoc.org/github.com/coreos/go-systemd)
-![minimum golang 1.5](https://img.shields.io/badge/golang-1.5%2B-orange.svg)
+![minimum golang 1.7](https://img.shields.io/badge/golang-1.7%2B-orange.svg)
 
 
 Go bindings to systemd. The project has several packages: