Add vet check to CI

Signed-off-by: Alexander Morozov <lk4d4@docker.com>

.travis.yml

@@ -5,4 +5,10 @@ go:
   - 1.5
   - tip
 
-script: go test ./...
+install:
+  - go get -d ./...
+  - go get golang.org/x/tools/cmd/vet
+
+script:
+  - go vet ./...
+  - go test ./...