all: update sync dependency

Change-Id: I9be8b83abcb17eb4e90b56cb99a800216a3c83f4
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2402
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

go.mod

@@ -13,7 +13,7 @@ require (
 	github.com/retr0h/go-gilt v0.0.0-20190206215556-f73826b37af2
 	github.com/spf13/cobra v0.0.3
 	github.com/spf13/pflag v1.0.3
-	golang.org/x/sync v0.0.0-20181108010431-42b317875d0f
+	golang.org/x/sync v0.0.0-20190423024810-112230192c58
 	golang.org/x/text v0.3.2
 	golang.org/x/tools v0.0.0-20181210225255-6a3e9aa2ab77
 	golang.org/x/xerrors v0.0.0-20190410155217-1f06c39b4373