Merge pull request #138 from airbnb/fix_ruby_2_x_support

Do not unregister the callback in the zk watcher

lib/synapse/service_watcher/zookeeper.rb

@@ -84,13 +84,12 @@ def watch
       return if @zk.nil?
       log.debug "synapse: setting watch at #{@discovery['path']}"
 
-      @watcher.unsubscribe unless @watcher.nil?
-      @watcher = @zk.register(@discovery['path'], &watcher_callback)
+      @watcher = @zk.register(@discovery['path'], &watcher_callback) unless @watcher
 
       # Verify that we actually set up the watcher.
       unless @zk.exists?(@discovery['path'], :watch => true)
         log.error "synapse: zookeeper watcher path #{@discovery['path']} does not exist!"
-        raise RuntimeError.new('could not set a ZK watch on a node that should exist')
+        zk_cleanup
       end
       log.debug "synapse: set watch at #{@discovery['path']}"
     end