Configure Travis to test Go1.6 (again) (#571)

diff --git a/.travis.yml b/.travis.yml
index 635293e..455fa66 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 sudo: false
 language: go
 go:
-- 1.9.x
+- 1.6.x
 - 1.10.x
 - 1.x