.travis.yml: bump Go versions and use '.x' to get latest patch versions

Change-Id: Ib536784b03c309592a8c046af6a62651310e7bc2
GitHub-Last-Rev: 24d3dd96324686f764ef7353864c14b8c08d598f
GitHub-Pull-Request: golang/gddo#589
Reviewed-on: https://go-review.googlesource.com/c/145239
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 5b9c0e1..8bbd568 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,7 @@
 language: go
 go:
-  - 1.9
+  - 1.11.x
+  - 1.10.x
 install:
   - |
     LATEST_SDK="$(curl -fsSL 'https://www.googleapis.com/storage/v1/b/appengine-sdks/o?prefix=featured%2F' |