Remove travis build/test using 1.8.

Change-Id: I0bb528467effb355c435ec55f792a027ffd41341
Reviewed-on: https://go-review.googlesource.com/86036
Reviewed-by: Tuo Shan <shantuo@google.com>
diff --git a/.travis.yml b/.travis.yml
index 03c46b4..5b9c0e1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,5 @@
 language: go
 go:
-  - 1.8
   - 1.9
 install:
   - |