update vendoring dependencies

vendor/github.com/docker/distribution/context/util.go

@@ -8,25 +8,17 @@ import (
 // since that time. If the key is not found, the value returned will be zero.
 // This is helpful when inferring metrics related to context execution times.
 func Since(ctx Context, key interface{}) time.Duration {
-	startedAtI := ctx.Value(key)
-	if startedAtI != nil {
-		if startedAt, ok := startedAtI.(time.Time); ok {
-			return time.Since(startedAt)
-		}
+	if startedAt, ok := ctx.Value(key).(time.Time); ok {
+		return time.Since(startedAt)
 	}
-
 	return 0
 }
 
 // GetStringValue returns a string value from the context. The empty string
 // will be returned if not found.
 func GetStringValue(ctx Context, key interface{}) (value string) {
-	stringi := ctx.Value(key)
-	if stringi != nil {
-		if valuev, ok := stringi.(string); ok {
-			value = valuev
-		}
+	if valuev, ok := ctx.Value(key).(string); ok {
+		value = valuev
 	}
-
 	return value
 }