temporary disable 1.6 on travis (#1198)

diff --git a/.travis.yml b/.travis.yml
index b3577c7..51f7d32 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,6 @@
 language: go
 
 go:
-  - 1.6.3
   - 1.7
   - 1.8