travis: switch to Go 1.9+1.10 (google#539)

dvyukov · web-flow · commit 4514465649b2 · 2018-03-20T14:34:44.000+01:00
1.9 is available for a long time, no point in testing in 1.9rc1.
1.10 is already released too.
diff --git a/.travis.yml b/.travis.yml
@@ -2,8 +2,8 @@ language: go
 dist: trusty
 
 go:
-  - 1.8
-  - 1.9rc1
+  - "1.9"
+  - "1.10"
 
 before_install:
   - echo $PATH
