vendor: update all dependencies

* Update all dependencies
  * Remove all `[[constraint]]` from Gopkg.toml
  * Add in the minimum number of `[[override]]` to build
  * Remove go get of github.com/inconshreveable/mousetrap as it is vendored
  * Update docs with new policy on constraints

vendor/golang.org/x/net/http2/transport.go

@@ -951,6 +951,9 @@ func (cc *ClientConn) awaitOpenSlotForRequest(req *http.Request) error {
 	for {
 		cc.lastActive = time.Now()
 		if cc.closed || !cc.canTakeNewRequestLocked() {
+			if waitingForConn != nil {
+				close(waitingForConn)
+			}
 			return errClientConnUnusable
 		}
 		if int64(len(cc.streams))+1 <= int64(cc.maxConcurrentStreams) {