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/README.md

@@ -1,6 +1,7 @@
 [![GoDoc](https://godoc.org/github.com/pkg/xattr?status.svg)](http://godoc.org/github.com/pkg/xattr)
 [![Go Report Card](https://goreportcard.com/badge/github.com/pkg/xattr)](https://goreportcard.com/report/github.com/pkg/xattr)
 [![Build Status](https://travis-ci.org/pkg/xattr.svg?branch=master)](https://travis-ci.org/pkg/xattr)
+[![Version](https://badge.fury.io/gh/pkg%2Fxattr.svg)](https://github.com/pkg/xattr/releases)
 
 xattr
 =====
@@ -8,6 +9,11 @@ Extended attribute support for Go (linux + darwin + freebsd).
 
 "Extended attributes are name:value pairs associated permanently with files and directories, similar to the environment strings associated with a process. An attribute may be defined or undefined. If it is defined, its value may be empty or non-empty." [See more...](https://en.wikipedia.org/wiki/Extended_file_attributes)
 
+`SetWithFlags` allows to additionally pass system flags to be forwarded to the underlying calls, FreeBSD does not support this and the parameter will be ignored.
+
+The `L` variants of all functions (`LGet/LSet/...`) are identical to `Get/Set/...` except that they
+do not reference a symlink that appears at the end of a path. See
+[GoDoc](http://godoc.org/github.com/pkg/xattr) for details.
 
 ### Example
 ```go
@@ -31,4 +37,9 @@ Extended attribute support for Go (linux + darwin + freebsd).
   if err = xattr.Remove(path, prefix+"test"); err != nil {
   	log.Fatal(err)
   }
+
+  // One can also specify the flags parameter to be passed to the OS.
+  if err := xattr.SetWithFlags(path, prefix+"test", []byte("test-attr-value"), xattr.XATTR_CREATE); err != nil {
+  	log.Fatal(err)
+  }
 ```