travis: run make verify-dependencies with Go 1.x (1.14.x)

Background: opencontainers#2073 (comment)

> switched back to running the vendoring in 1.14. If we vendor with go
> 1.13, go build will fail on go 1.14, making the tests fil.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.travis.yml

@@ -7,7 +7,7 @@ go:
 
 matrix:
   include:
-    - go: 1.13.x
+    - go: 1.x
       name: "verify-dependencies"
       script:
         - make verify-dependencies