Stop testing against go 1.8

Go 1.8 doesn't have the `testing.Helper()` method
https://travis-ci.org/github/hub/jobs/481181084

.travis.yml

@@ -8,7 +8,6 @@ go:
   - 1.11.x
   - 1.10.x
   - 1.9.x
-  - 1.8.x
 script: make test-all
 install: script/bootstrap
 after_success: script/publish-release

CONTRIBUTING.md

@@ -9,7 +9,7 @@ This project adheres to a [Code of Conduct][code-of-conduct]. By participating,
 
 You will need:
 
-1. Go 1.8+
+1. Go 1.9+
 1. Ruby 1.9+ with Bundler
 2. git 1.8+
 3. tmux & zsh (optional) - for running shell completion tests

README.md

@@ -86,7 +86,7 @@ make install prefix=/usr/local
 Prerequisites for compilation are:
 
 * `make`
-* [Go 1.8+](http://golang.org/doc/install)
+* [Go 1.9+](http://golang.org/doc/install)
 * Ruby 1.9+ with Bundler - for generating man pages
 
 Aliasing