Drop support for Go 1.8

.travis.yml

@@ -5,7 +5,6 @@ os:
   - osx
 
 go:
-  - 1.8.x
   - 1.9.x
   - 1.10.x
   - 1.11.x

README.md

@@ -12,7 +12,7 @@ mastotool offers a collection of tools to work with your Mastodon account
 
 ### From source
 
-Make sure you have a working Go environment (Go 1.8 or higher is required).
+Make sure you have a working Go environment (Go 1.9 or higher is required).
 See the [install instructions](http://golang.org/doc/install.html).
 
 Compiling mastotool is easy, simply run: