Add go vet check (#430)

* Add go vet check

* Replace go vet with go tool vet to avoid printing exit message

hack/verify-govet.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+find_files() {
+  find . -not \( \
+      \( \
+        -wholename '*/vendor/*' \
+      \) -prune \
+    \) -name '*.go'
+}
+
+find_files | grep -v '.glide/cache' | xargs -I@ bash -c "go tool vet @"