Update mattermost library (#2152)

* Update mattermost library

* Fix linting

vendor/github.com/hashicorp/yamux/README.md

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+# Yamux
+
+Yamux (Yet another Multiplexer) is a multiplexing library for Golang.
+It relies on an underlying connection to provide reliability
+and ordering, such as TCP or Unix domain sockets, and provides
+stream-oriented multiplexing. It is inspired by SPDY but is not
+interoperable with it.
+
+Yamux features include:
+
+* Bi-directional streams
+  * Streams can be opened by either client or server
+  * Useful for NAT traversal
+  * Server-side push support
+* Flow control
+  * Avoid starvation
+  * Back-pressure to prevent overwhelming a receiver
+* Keep Alives
+  * Enables persistent connections over a load balancer
+* Efficient
+  * Enables thousands of logical streams with low overhead
+
+## Documentation
+
+For complete documentation, see the associated [Godoc](http://godoc.org/github.com/hashicorp/yamux).
+
+## Specification
+
+The full specification for Yamux is provided in the `spec.md` file.
+It can be used as a guide to implementors of interoperable libraries.
+
+## Usage
+
+Using Yamux is remarkably simple:
+
+```go
+
+func client() {
+    // Get a TCP connection
+    conn, err := net.Dial(...)
+    if err != nil {
+        panic(err)
+    }
+
+    // Setup client side of yamux
+    session, err := yamux.Client(conn, nil)
+    if err != nil {
+        panic(err)
+    }
+
+    // Open a new stream
+    stream, err := session.Open()
+    if err != nil {
+        panic(err)
+    }
+
+    // Stream implements net.Conn
+    stream.Write([]byte("ping"))
+}
+
+func server() {
+    // Accept a TCP connection
+    conn, err := listener.Accept()
+    if err != nil {
+        panic(err)
+    }
+
+    // Setup server side of yamux
+    session, err := yamux.Server(conn, nil)
+    if err != nil {
+        panic(err)
+    }
+
+    // Accept a stream
+    stream, err := session.Accept()
+    if err != nil {
+        panic(err)
+    }
+
+    // Listen for a message
+    buf := make([]byte, 4)
+    stream.Read(buf)
+}
+
+```
+