This version fixes occasional NPE on startup:

```
WARN  com.netflix.spectator.gc.GcLogger - exception thrown by event listener
java.lang.NullPointerException
        at com.netflix.spectator.http.RxHttp.getServersForVip(RxHttp.java:561) ~[spectator-nflx-plugin-0.18.jar:?]
        at com.netflix.spectator.http.RxHttp.getServers(RxHttp.java:542) ~[spectator-nflx-plugin-0.18.jar:?]
        at com.netflix.spectator.http.RxHttp.post(RxHttp.java:171) ~[spectator-nflx-plugin-0.18.jar:?]
        at com.netflix.spectator.http.RxHttp.postJson(RxHttp.java:188) ~[spectator-nflx-plugin-0.18.jar:?]
        at com.netflix.spectator.nflx.ChronosGcEventListener.sendToChronos(ChronosGcEventListener.java:109) ~[spectator-nflx-plugin-0.18.jar:?]
        at com.netflix.spectator.nflx.ChronosGcEventListener.onComplete(ChronosGcEventListener.java:161) ~[spectator-nflx-plugin-0.18.jar:?]
        at com.netflix.spectator.nflx.ChronosGcEventListener.onComplete(ChronosGcEventListener.java:170) ~[spectator-nflx-plugin-0.18.jar:?]
        at com.netflix.spectator.gc.GcLogger.processGcEvent(GcLogger.java:212) [spectator-ext-gc-0.18.jar:?]
        at com.netflix.spectator.gc.GcLogger.access$100(GcLogger.java:48) [spectator-ext-gc-0.18.jar:?]
        at com.netflix.spectator.gc.GcLogger$GcNotificationListener.handleNotification(GcLogger.java:225) [spectator-ext-gc-0.18.jar:?]
        at sun.management.NotificationEmitterSupport.sendNotification(NotificationEmitterSupport.java:156) [?:1.8.0_25]
        at sun.management.GarbageCollectorImpl.createGCNotification(GarbageCollectorImpl.java:147) [?:1.8.0_25]
```