build: always run go vet

This ensures 'make test' finds all errors that remote CI would find.
Go 1.7 vet reports a false positive in package log, add a workaround.

log/handler.go

@@ -106,11 +106,16 @@ func CallerFileHandler(h Handler) Handler {
 // the context with key "fn".
 func CallerFuncHandler(h Handler) Handler {
 	return FuncHandler(func(r *Record) error {
-		r.Ctx = append(r.Ctx, "fn", fmt.Sprintf("%+n", r.Call))
+		r.Ctx = append(r.Ctx, "fn", formatCall("%+n", r.Call))
 		return h.Log(r)
 	})
 }
 
+// This function is here to please go vet on Go < 1.8.
+func formatCall(format string, c stack.Call) string {
+	return fmt.Sprintf(format, c)
+}
+
 // CallerStackHandler returns a Handler that adds a stack trace to the context
 // with key "stack". The stack trace is formated as a space separated list of
 // call sites inside matching []'s. The most recent call site is listed first.