Update dependencies, enable pruning for vendor/

So, `dep` got an nice new feature to remove tests and non-go files from
`vendor/`, and this brings the size of the vendor directory from ~300MiB
down to ~20MiB. We don that now.

vendor/github.com/pkg/xattr/go.mod

@@ -1 +1,3 @@
-module "github.com/pkg/xattr"
+module github.com/pkg/xattr
+
+require golang.org/x/sys v0.0.0-20180525142821-c11f84a56e43