Drop support for Go 1.6 and lower (go-sql-driver#696)

* Drop support for Go 1.6 and lower

* Remove cloneTLSConfig for legacy Go versions

Author: julienschmidt

.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+)
 
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