vendor: fix dep warning and update yamux dependency

I got following warning after upgrading dep tool:

Warning: the following project(s) have [[constraint]] stanzas in Gopkg.toml:

  ✗  github.com/hashicorp/yamux

However, these projects are not direct dependencies of the current project:
they are not imported in any .go files, nor are they in the 'required' list in
Gopkg.toml. Dep only applies [[constraint]] rules to direct dependencies, so
these rules will have no effect.

Either import/require packages from these projects so that they become direct
dependencies, or convert each [[constraint]] to an [[override]] to enforce rules
on these projects, if they happen to be transitive dependencies,

So let's convert constraint to override over yamux. In the meanwhile,
update the yamux vendor. Full commit list:

4c2fe0d (origin/b-consul-3040) Dont output keepalive error when the session is closed
f21aae5 Make sure to drain the timer channel on defer, and a clarifying comment
601ccd8 Make receive window update logic a bit cleaner
02d320c Uses timer pool in sendNoWait, like in waitForSendErr
cf433c5 window update unit test for partial read; benchmark large buffer
ca8dfd0 improve memory utilization in receive buffer, fix flow control
683f491 Fix race around read and write deadlines in Stream (#52)
40b86b2 Add public session CloseChan method (#44)

Note that commit 4c2fe0d might also help kata-containers/agent/issues/231.

Signed-off-by: Peng Tao <bergwolf@gmail.com>

Gopkg.toml

@@ -70,11 +70,11 @@
   name = "github.com/safchain/ethtool"
   revision = "79559b488d8848b53a8e34c330140c3fc37ee246"
 
+[[override]]
+  branch = "master"
+  name = "github.com/hashicorp/yamux"
+
 [prune]
   non-go = true
   go-tests = true
   unused-packages = true
-
-[[constraint]]
-  branch = "master"
-  name = "github.com/hashicorp/yamux"