all: remove 1.8 and 1.9 from Travis builds

1.8 no longer compiles, and 1.9 is no longer supported.

Change-Id: I85cd9fb22dfac97486cf4e4e02cdc936af41cc31
Reviewed-on: https://go-review.googlesource.com/c/144179
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/.travis.yml b/.travis.yml
index b3fdca9..50553eb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,6 @@
 sudo: false
 language: go
 go:
-  - 1.8.x
-  - 1.9.x
   - 1.10.x
   - 1.11.x
   - master