Bump aws sdk to v1.15.11

This is the latest official release for this dependency

Signed-off-by: Ryan Abrams <rdabrams@gmail.com>

vendor/github.com/aws/aws-sdk-go/private/protocol/rest/unmarshal.go

@@ -198,7 +198,11 @@ func unmarshalHeader(v reflect.Value, header string, tag reflect.StructTag) erro
 		}
 		v.Set(reflect.ValueOf(&f))
 	case *time.Time:
-		t, err := time.Parse(RFC822, header)
+		format := tag.Get("timestampFormat")
+		if len(format) == 0 {
+			format = protocol.RFC822TimeFormatName
+		}
+		t, err := protocol.ParseTime(format, header)
 		if err != nil {
 			return err
 		}