drop Go 1.8 support (#936)

* drop Go 1.8 support

* travis: add Go 1.12.x

.travis.yml

@@ -1,10 +1,10 @@
 sudo: false
 language: go
 go:
-  - 1.8.x
   - 1.9.x
   - 1.10.x
   - 1.11.x
+  - 1.12.x
   - master
 
 before_install:

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 1.8 or higher. We aim to support the 3 latest versions of Go.
+  * Go 1.9 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+)
 
 ---------------------------------------