Update to jetcd 0.5.*

[psychoveter]

Changes in jetcd 0.5.4 are not compatible with current version.
E.g. it seems io.etcd.jetcd.Observers is missed in new jetcd.

Exception in thread "Thread-2" java.lang.NoClassDefFoundError: io/etcd/jetcd/Observers
at io.etcd.recipes.common.LeaseUtils.keepAlive(LeaseExtensions.kt:33)
at io.etcd.recipes.common.LeaseUtils.keepAliveWith(LeaseExtensions.kt:29)
at io.etcd.recipes.common.KeepAliveUtils.putValuesWithKeepAlive-Q15Xa8I(KeepAliveExtensions.kt:67)