build(deps): bump github.com/coreos/etcd

Bumps [github.com/coreos/etcd](https://github.com/coreos/etcd) from 3.3.17+incompatible to 3.3.18+incompatible.
- [Release notes](https://github.com/coreos/etcd/releases)
- [Changelog](https://github.com/etcd-io/etcd/blob/master/CHANGELOG-3.4.md)
- [Commits](etcd-io/etcd@v3.3.17...v3.3.18)

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

go.mod

@@ -5,7 +5,7 @@ go 1.13
 require (
 	github.com/allegro/bigcache v1.2.1
 	github.com/bradfitz/gomemcache v0.0.0-20190913173617-a41fca850d0b
-	github.com/coreos/etcd v3.3.17+incompatible
+	github.com/coreos/etcd v3.3.18+incompatible
 	github.com/dgraph-io/ristretto v0.0.0-20191010170704-2ba187ef9534
 	github.com/go-redis/redis/v7 v7.0.0-beta.4
 	github.com/jonboulle/clockwork v0.1.0 // indirect