Drop support for Go 1.6 and lower (#696)

* Drop support for Go 1.6 and lower

* Remove cloneTLSConfig for legacy Go versions

.travis.yml

@@ -1,8 +1,6 @@
 sudo: false
 language: go
 go:
-  - 1.5
-  - 1.6
   - 1.7
   - 1.8
   - 1.9

README.md

@@ -40,7 +40,7 @@ A MySQL-Driver for Go's [database/sql](https://golang.org/pkg/database/sql/) pac
   * Optional placeholder interpolation
 
 ## Requirements
-  * Go 1.5 or higher
+  * Go 1.7 or higher. We aim to support the 3 latest versions of Go.
   * MySQL (4.1+), MariaDB, Percona Server, Google CloudSQL or Sphinx (2.2.3+)
 
 ---------------------------------------

utils_legacy.go

@@ -1,18 +0,0 @@
-// Go MySQL Driver - A MySQL-Driver for Go's database/sql package
-//
-// Copyright 2017 The Go-MySQL-Driver Authors. All rights reserved.
-//
-// This Source Code Form is subject to the terms of the Mozilla Public
-// License, v. 2.0. If a copy of the MPL was not distributed with this file,
-// You can obtain one at http://mozilla.org/MPL/2.0/.
-
-// +build !go1.7
-
-package mysql
-
-import "crypto/tls"
-
-func cloneTLSConfig(c *tls.Config) *tls.Config {
-	clone := *c
-	return &clone
-}