Skip plugin checks on hosted Elasticsearch

The AWS Elasticsearch service does not expose the list of installed
plugins, so our plugin checking code breaks.

Fortunately, for now, it has the plugin we need already installed.

Note that in our testing, Pelias performs significantly worse on the
hosted Elasticsearch compared to regular EC2 instances.
See pelias/pelias#402 (comment)
for more details

Fixes #213

scripts/check_plugins.js

@@ -28,6 +28,13 @@ client.nodes.info( null, function( err, res ){
   for( var uid in res.nodes ){
 
     var node = res.nodes[uid];
+
+    // Amazon's hosted Elasticsearch does not have the plugins property
+    // but has the plugins we need
+    if (!node.plugins) {
+      continue;
+    }
+
     console.log( util.format( "\033[1;37mnode '%s' [%s]\033[0m", node.name, uid ) );
 
     // per node failures