build(deps): bump github.com/urfave/cli from 1.20.0 to 1.21.0 (#5928)

Bumps [github.com/urfave/cli](https://github.com/urfave/cli) from 1.20.0 to 1.21.0.
- [Release notes](https://github.com/urfave/cli/releases)
- [Changelog](https://github.com/urfave/cli/blob/master/CHANGELOG.md)
- [Commits](https://github.com/urfave/cli/compare/v1.20.0...v1.21.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

go.mod

@@ -36,7 +36,7 @@ require (
 	github.com/syncthing/notify v0.0.0-20190709140112-69c7a957d3e2
 	github.com/syndtr/goleveldb v1.0.1-0.20190318030020-c3a204f8e965
 	github.com/thejerf/suture v3.0.2+incompatible
-	github.com/urfave/cli v1.20.0
+	github.com/urfave/cli v1.21.0
 	github.com/vitrun/qart v0.0.0-20160531060029-bf64b92db6b0
 	golang.org/x/crypto v0.0.0-20190611184440-5c40567a22f8
 	golang.org/x/net v0.0.0-20190613194153-d28f0bde5980