go versions are strings

.travis.yml

 language: go
 go:
-- 1.8
-- 1.9
-- 1.10
+- "1.8"
+- "1.9"
+- "1.10"
 install:
 - go get -u github.com/stretchr/testify/assert
 - go get -u github.com/PuerkitoBio/purell